From 2803d149bc8f340dbe5f82193c7929760f9321c4 Mon Sep 17 00:00:00 2001
From: Mehdi <mehdi@blagui.com>
Date: Sat, 14 Feb 2026 18:10:51 +0000
Subject: [PATCH 1/3] feat(renderer): add i18n locale support and language
 selector

---
 package-lock.json                             |  81 ++++++++++-
 package.json                                  |   2 +
 src/renderer/components/App.tsx               |  14 +-
 src/renderer/components/ConfigTab.tsx         | 103 +++++++-------
 src/renderer/components/DarkModeToggle.tsx    |   6 +-
 src/renderer/components/FileTree.tsx          |  19 ++-
 src/renderer/components/LanguageSelector.tsx  |  42 ++++++
 src/renderer/components/ProcessedTab.tsx      |  38 +++---
 src/renderer/components/SourceTab.tsx         |  32 ++---
 src/renderer/components/TabBar.tsx            |  11 +-
 src/renderer/context/AppContext.tsx           |  23 ++--
 src/renderer/i18n/index.ts                    |  33 +++++
 src/renderer/i18n/locales/de/common.json      | 127 ++++++++++++++++++
 src/renderer/i18n/locales/en/common.json      | 127 ++++++++++++++++++
 src/renderer/i18n/locales/es/common.json      | 127 ++++++++++++++++++
 src/renderer/i18n/locales/fr/common.json      | 127 ++++++++++++++++++
 src/renderer/i18n/settings.ts                 |  59 ++++++++
 src/renderer/index.tsx                        |   1 +
 tests/catalog.md                              |  68 +++++-----
 tests/e2e/electron-process-flow.spec.ts       |  19 +++
 tests/setup.ts                                |   8 ++
 .../components/language-selector.test.tsx     |  39 ++++++
 tests/unit/i18n/locales-parity.test.ts        |  43 ++++++
 23 files changed, 1003 insertions(+), 146 deletions(-)
 create mode 100644 src/renderer/components/LanguageSelector.tsx
 create mode 100644 src/renderer/i18n/index.ts
 create mode 100644 src/renderer/i18n/locales/de/common.json
 create mode 100644 src/renderer/i18n/locales/en/common.json
 create mode 100644 src/renderer/i18n/locales/es/common.json
 create mode 100644 src/renderer/i18n/locales/fr/common.json
 create mode 100644 src/renderer/i18n/settings.ts
 create mode 100644 tests/unit/components/language-selector.test.tsx
 create mode 100644 tests/unit/i18n/locales-parity.test.ts

diff --git a/package-lock.json b/package-lock.json
index 785db75..0dcb467 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -15,11 +15,13 @@
         "clsx": "^2.1.0",
         "electron-store": "^11.0.2",
         "electron-updater": "^6.7.3",
+        "i18next": "^25.8.7",
         "minimatch": "^10.1.2",
         "path-browserify": "^1.0.1",
         "process": "^0.11.10",
         "react": "^19.2.4",
         "react-dom": "^19.2.4",
+        "react-i18next": "^16.5.4",
         "react-router-dom": "^7.13.0",
         "tiktoken": "^1.0.11",
         "yaml": "^2.3.4"
@@ -1935,7 +1937,6 @@
       "version": "7.28.6",
       "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.28.6.tgz",
       "integrity": "sha512-05WQkdpL9COIMz4LjTxGpPNCdlpyimKppYNoJ5Di5EUObifl8t4tuLuUBBZEpoLYOmfvIWrsp9fCl0HoPRVTdA==",
-      "dev": true,
       "license": "MIT",
       "engines": {
         "node": ">=6.9.0"
@@ -10779,6 +10780,15 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/html-parse-stringify": {
+      "version": "3.0.1",
+      "resolved": "https://registry.npmjs.org/html-parse-stringify/-/html-parse-stringify-3.0.1.tgz",
+      "integrity": "sha512-KknJ50kTInJ7qIScF3jeaFRpMpE8/lfiTdzf/twXyPBLAGrLRTmkz3AdTnKeh40X8k9L2fdYwEp/42WGXIRGcg==",
+      "license": "MIT",
+      "dependencies": {
+        "void-elements": "3.1.0"
+      }
+    },
     "node_modules/http-cache-semantics": {
       "version": "4.2.0",
       "resolved": "https://registry.npmjs.org/http-cache-semantics/-/http-cache-semantics-4.2.0.tgz",
@@ -10854,6 +10864,37 @@
         "url": "https://github.com/sponsors/typicode"
       }
     },
+    "node_modules/i18next": {
+      "version": "25.8.7",
+      "resolved": "https://registry.npmjs.org/i18next/-/i18next-25.8.7.tgz",
+      "integrity": "sha512-ttxxc5+67S/0hhoeVdEgc1lRklZhdfcUSEPp1//uUG2NB88X3667gRsDar+ZWQFdysnOsnb32bcoMsa4mtzhkQ==",
+      "funding": [
+        {
+          "type": "individual",
+          "url": "https://locize.com"
+        },
+        {
+          "type": "individual",
+          "url": "https://locize.com/i18next.html"
+        },
+        {
+          "type": "individual",
+          "url": "https://www.i18next.com/how-to/faq#i18next-is-awesome.-how-can-i-support-the-project"
+        }
+      ],
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.28.4"
+      },
+      "peerDependencies": {
+        "typescript": "^5"
+      },
+      "peerDependenciesMeta": {
+        "typescript": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/iconv-corefoundation": {
       "version": "1.1.7",
       "resolved": "https://registry.npmjs.org/iconv-corefoundation/-/iconv-corefoundation-1.1.7.tgz",
@@ -16451,6 +16492,33 @@
         "react": "^19.2.4"
       }
     },
+    "node_modules/react-i18next": {
+      "version": "16.5.4",
+      "resolved": "https://registry.npmjs.org/react-i18next/-/react-i18next-16.5.4.tgz",
+      "integrity": "sha512-6yj+dcfMncEC21QPhOTsW8mOSO+pzFmT6uvU7XXdvM/Cp38zJkmTeMeKmTrmCMD5ToT79FmiE/mRWiYWcJYW4g==",
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.28.4",
+        "html-parse-stringify": "^3.0.1",
+        "use-sync-external-store": "^1.6.0"
+      },
+      "peerDependencies": {
+        "i18next": ">= 25.6.2",
+        "react": ">= 16.8.0",
+        "typescript": "^5"
+      },
+      "peerDependenciesMeta": {
+        "react-dom": {
+          "optional": true
+        },
+        "react-native": {
+          "optional": true
+        },
+        "typescript": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/react-is": {
       "version": "17.0.2",
       "resolved": "https://registry.npmjs.org/react-is/-/react-is-17.0.2.tgz",
@@ -18709,7 +18777,7 @@
       "version": "5.9.3",
       "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.9.3.tgz",
       "integrity": "sha512-jl1vZzPDinLr9eUt3J/t7V6FgNEw9QjvBPdysz9KfQDD41fQrC2Y4vKQdiaUpFT4bXlb1RHhLpp8wtm6M5TgSw==",
-      "dev": true,
+      "devOptional": true,
       "license": "Apache-2.0",
       "bin": {
         "tsc": "bin/tsc",
@@ -18992,6 +19060,15 @@
         "node": ">=0.6.0"
       }
     },
+    "node_modules/void-elements": {
+      "version": "3.1.0",
+      "resolved": "https://registry.npmjs.org/void-elements/-/void-elements-3.1.0.tgz",
+      "integrity": "sha512-Dhxzh5HZuiHQhbvTW9AMetFfBHDMYpo23Uo9btPXgdYP+3T5S+p+jgNy7spra+veYhBP2dCSgxR/i2Y02h5/6w==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.10.0"
+      }
+    },
     "node_modules/w3c-xmlserializer": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/w3c-xmlserializer/-/w3c-xmlserializer-5.0.0.tgz",
diff --git a/package.json b/package.json
index f70d617..efa435c 100644
--- a/package.json
+++ b/package.json
@@ -133,11 +133,13 @@
     "clsx": "^2.1.0",
     "electron-store": "^11.0.2",
     "electron-updater": "^6.7.3",
+    "i18next": "^25.8.7",
     "minimatch": "^10.1.2",
     "path-browserify": "^1.0.1",
     "process": "^0.11.10",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
+    "react-i18next": "^16.5.4",
     "react-router-dom": "^7.13.0",
     "tiktoken": "^1.0.11",
     "yaml": "^2.3.4"
diff --git a/src/renderer/components/App.tsx b/src/renderer/components/App.tsx
index e4c4546..9baf3f1 100755
--- a/src/renderer/components/App.tsx
+++ b/src/renderer/components/App.tsx
@@ -1,16 +1,20 @@
 import React from 'react';
+import { useTranslation } from 'react-i18next';
 
 import { AppProvider, useApp } from '../context/AppContext';
 import { DarkModeProvider } from '../context/DarkModeContext';
+import '../i18n';
 
 import ConfigTab from './ConfigTab';
 import DarkModeToggle from './DarkModeToggle';
+import LanguageSelector from './LanguageSelector';
 import ProcessedTab from './ProcessedTab';
 import SourceTab from './SourceTab';
 import TabBar from './TabBar';
 
 const ErrorBanner = () => {
   const { appError, dismissError } = useApp();
+  const { t } = useTranslation();
 
   if (!appError) return null;
 
@@ -20,7 +24,7 @@ const ErrorBanner = () => {
       <button
         onClick={dismissError}
         className='ml-4 shrink-0 text-red-600 dark:text-red-300 hover:text-red-800 dark:hover:text-red-100'
-        aria-label='Dismiss error'
+        aria-label={t('app.dismissError')}
       >
         <svg className='h-4 w-4' fill='none' stroke='currentColor' viewBox='0 0 24 24'>
           <path strokeLinecap='round' strokeLinejoin='round' strokeWidth={2} d='M6 18L18 6M6 6l12 12' />
@@ -31,6 +35,7 @@ const ErrorBanner = () => {
 };
 
 const AppContent = () => {
+  const { t } = useTranslation();
   const {
     activeTab,
     rootPath,
@@ -59,6 +64,7 @@ const AppContent = () => {
         <div className='w-full border-b border-gray-300 dark:border-gray-700 flex justify-between items-center bg-gray-100 dark:bg-gray-800 transition-colors duration-200'>
           <TabBar activeTab={activeTab} onTabChange={switchTab} />
           <div className='flex items-center pr-4 space-x-2'>
+            <LanguageSelector />
             <DarkModeToggle />
             <button
               onClick={() => {
@@ -67,12 +73,12 @@ const AppContent = () => {
                 );
               }}
               className='flex items-center hover:text-blue-700 dark:hover:text-blue-500 cursor-pointer bg-transparent border-0 text-gray-900 dark:text-gray-100'
-              title='View on GitHub'
+              title={t('app.github')}
             >
               <div className='h-8 w-8 mr-2 flex items-center justify-center'>
                 <img
                   src='icon.png'
-                  alt='AI Code Fusion'
+                  alt={t('app.name')}
                   className='h-8 w-8'
                   onError={(event: React.SyntheticEvent<HTMLImageElement, Event>) => {
                     console.error('Failed to load icon.png');
@@ -101,7 +107,7 @@ const AppContent = () => {
                 </svg>
               </div>
               <div className='flex items-center'>
-                <h1 className='text-2xl font-bold dark:text-white'>AI Code Fusion</h1>
+                <h1 className='text-2xl font-bold dark:text-white'>{t('app.name')}</h1>
                 <svg
                   className='ml-2 w-5 h-5 text-gray-600 dark:text-gray-400'
                   fill='currentColor'
diff --git a/src/renderer/components/ConfigTab.tsx b/src/renderer/components/ConfigTab.tsx
index 5809775..1cb1c10 100755
--- a/src/renderer/components/ConfigTab.tsx
+++ b/src/renderer/components/ConfigTab.tsx
@@ -1,4 +1,5 @@
 import React, { useCallback, useEffect, useReducer, useRef, useState } from 'react';
+import { useTranslation } from 'react-i18next';
 import yaml from 'yaml';
 
 import { normalizeExportFormat } from '../../utils/export-format';
@@ -81,7 +82,7 @@ const getProviderValidationErrors = (providerFields: {
   providerModel: string;
   providerApiKey: string;
   providerBaseUrl: string;
-}): string[] => {
+}, translate: (key: string) => string): string[] => {
   if (!hasProviderInput(providerFields)) {
     return [];
   }
@@ -90,28 +91,28 @@ const getProviderValidationErrors = (providerFields: {
   const { providerId, providerModel, providerApiKey, providerBaseUrl } = providerFields;
 
   if (!providerId) {
-    errors.push('Select a provider.');
+    errors.push(translate('config.validation.selectProvider'));
   }
 
   if (!providerModel.trim()) {
-    errors.push('Model is required.');
+    errors.push(translate('config.validation.modelRequired'));
   }
 
   const selectedProviderOption = PROVIDER_OPTIONS.find(
     (providerOption) => providerOption.id === providerId
   );
   if (selectedProviderOption?.requiresApiKey && !providerApiKey.trim()) {
-    errors.push('API key is required for this provider.');
+    errors.push(translate('config.validation.apiKeyRequired'));
   }
 
   if (providerBaseUrl.trim()) {
     try {
       const parsedUrl = new URL(providerBaseUrl.trim());
       if (!['http:', 'https:'].includes(parsedUrl.protocol)) {
-        errors.push('Base URL must use http or https.');
+        errors.push(translate('config.validation.baseUrlProtocol'));
       }
     } catch {
-      errors.push('Base URL must be a valid URL.');
+      errors.push(translate('config.validation.baseUrlValid'));
     }
   }
 
@@ -206,6 +207,7 @@ const initialFormState: ConfigFormState = {
 };
 
 const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
+  const { t } = useTranslation();
   const { rootPath, selectDirectory, switchTab } = useApp();
   const [formState, dispatch] = useReducer(configFormReducer, initialFormState);
   const [isSaved, setIsSaved] = useState(false);
@@ -274,14 +276,14 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
             providerApiKey: state.providerApiKey,
             providerBaseUrl: state.providerBaseUrl,
           };
-          const validationErrors = getProviderValidationErrors(providerFields);
+          const validationErrors = getProviderValidationErrors(providerFields, t);
           hasProviderValidationErrors = validationErrors.length > 0;
 
           if (hasProviderValidationErrors) {
             setProviderValidationErrors(validationErrors);
             setProviderTestResult({
               ok: false,
-              message: 'Fix provider settings before saving.',
+              message: t('config.providerFixBeforeSaving'),
             });
           }
 
@@ -326,7 +328,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
         console.error('Error updating config:', error);
       }
     },
-    [aiSurfacesEnabled, configContent, onConfigChange]
+    [aiSurfacesEnabled, configContent, onConfigChange, t]
   );
 
   // Auto-save on checkbox/select changes (not text fields - those save on blur/button)
@@ -354,7 +356,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
     if (!aiSurfacesEnabled) {
       setProviderTestResult({
         ok: false,
-        message: 'Provider connection testing is disabled outside dev mode.',
+        message: t('config.providerTestDisabled'),
       });
       return;
     }
@@ -365,25 +367,25 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
       providerApiKey: formState.providerApiKey,
       providerBaseUrl: formState.providerBaseUrl,
     };
-    const validationErrors = getProviderValidationErrors(providerFields);
+    const validationErrors = getProviderValidationErrors(providerFields, t);
     if (validationErrors.length > 0) {
       setProviderValidationErrors(validationErrors);
       setProviderTestResult({
         ok: false,
-        message: 'Fix provider settings before testing the connection.',
+        message: t('config.providerFixBeforeTesting'),
       });
       return;
     }
 
     if (!formState.providerId) {
-      setProviderValidationErrors(['Select a provider.']);
+      setProviderValidationErrors([t('config.validation.selectProvider')]);
       return;
     }
 
     if (!appWindow.electronAPI?.testProviderConnection) {
       setProviderTestResult({
         ok: false,
-        message: 'Provider connection testing is unavailable in this build.',
+        message: t('config.providerUnavailable'),
       });
       return;
     }
@@ -405,9 +407,9 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
     } catch (error) {
       setProviderTestResult({
         ok: false,
-        message: `Connection test failed: ${
-          error instanceof Error ? error.message : 'Unknown error'
-        }`,
+        message: t('config.connectionTestFailed', {
+          message: error instanceof Error ? error.message : 'Unknown error',
+        }),
       });
     } finally {
       setIsTestingProviderConnection(false);
@@ -435,12 +437,13 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
             className='grow border border-gray-300 dark:border-gray-600 dark:bg-gray-700 dark:text-white px-3 py-2 shadow-sm focus:border-blue-500 focus:outline-none focus:ring-blue-500 cursor-pointer'
             value={rootPath}
             readOnly
-            placeholder='Select a root folder'
+            placeholder={t('config.selectRootFolderPlaceholder')}
             onClick={handleFolderSelect}
-            title='Click to browse for a directory'
+            title={t('config.browseDirectoryTitle')}
           />
           <button
             onClick={handleFolderSelect}
+            data-testid='select-folder-button'
             className='ml-2 inline-flex items-center border border-transparent bg-blue-600 px-4 py-2 text-sm font-medium text-white shadow-sm hover:bg-blue-700 focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2'
           >
             <svg
@@ -457,7 +460,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                 d='M3 7v10a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-6l-2-2H5a2 2 0 00-2 2z'
               />
             </svg>
-            Select Folder
+            {t('config.selectFolder')}
           </button>
         </div>
       </div>
@@ -468,7 +471,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
             {/* File Filtering section */}
             <div className='rounded border border-gray-200 dark:border-gray-700 bg-white dark:bg-gray-700 p-4'>
               <h4 className='mb-2 text-sm font-semibold text-gray-700 dark:text-gray-300'>
-                File Filtering
+                {t('config.fileFilteringTitle')}
               </h4>
 
               <div className='space-y-2'>
@@ -484,7 +487,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='use-custom-includes'
                     className='ml-2 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Filter by file extensions
+                    {t('config.filterByExtensions')}
                   </label>
                 </div>
 
@@ -500,7 +503,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='use-custom-excludes'
                     className='ml-2 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Use exclude patterns
+                    {t('config.useExcludePatterns')}
                   </label>
                 </div>
 
@@ -516,7 +519,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='use-gitignore'
                     className='ml-2 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Apply .gitignore rules
+                    {t('config.applyGitignoreRules')}
                   </label>
                 </div>
 
@@ -532,7 +535,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='enable-secret-scanning'
                     className='ml-2 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Scan content for secrets
+                    {t('config.scanSecrets')}
                   </label>
                 </div>
 
@@ -548,7 +551,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='exclude-suspicious-files'
                     className='ml-2 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Exclude suspicious files
+                    {t('config.excludeSuspiciousFiles')}
                   </label>
                 </div>
               </div>
@@ -557,7 +560,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
             {/* Output Formatting section */}
             <div className='rounded border border-gray-200 dark:border-gray-700 bg-white dark:bg-gray-700 p-4'>
               <h4 className='mb-2 text-sm font-semibold text-gray-700 dark:text-gray-300'>
-                Output Formatting
+                {t('config.outputFormattingTitle')}
               </h4>
 
               <div className='space-y-2'>
@@ -573,7 +576,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='include-tree-view'
                     className='ml-2 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Include file tree in output
+                    {t('config.includeFileTree')}
                   </label>
                 </div>
 
@@ -589,7 +592,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='show-token-count'
                     className='ml-2 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Display token counts
+                    {t('config.displayTokenCounts')}
                   </label>
                 </div>
 
@@ -598,16 +601,17 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='export-format'
                     className='mb-1 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Export format
+                    {t('config.exportFormat')}
                   </label>
                   <select
                     id='export-format'
                     value={formState.exportFormat}
                     onChange={(event) => setField('exportFormat', normalizeExportFormat(event.target.value))}
+                    data-testid='export-format-select'
                     className='w-full rounded border border-gray-300 dark:border-gray-600 dark:bg-gray-700 dark:text-white px-2 py-1 text-sm focus:border-blue-500 focus:outline-none focus:ring-blue-500'
                   >
-                    <option value='markdown'>Markdown</option>
-                    <option value='xml'>XML</option>
+                    <option value='markdown'>{t('config.exportFormatMarkdown')}</option>
+                    <option value='xml'>{t('config.exportFormatXml')}</option>
                   </select>
                 </div>
               </div>
@@ -617,10 +621,10 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
           {aiSurfacesEnabled && (
             <div className='rounded border border-gray-200 dark:border-gray-700 bg-white dark:bg-gray-700 p-4 mt-4'>
               <h4 className='mb-2 text-sm font-semibold text-gray-700 dark:text-gray-300'>
-                Provider Setup Assistant
+                {t('config.providerSetupTitle')}
               </h4>
               <p className='mb-3 text-xs text-gray-500 dark:text-gray-400'>
-                Configure a model provider and run a connection test before saving.
+                {t('config.providerSetupDescription')}
               </p>
 
               <div className='grid grid-cols-1 md:grid-cols-2 gap-3'>
@@ -629,7 +633,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='provider-id'
                     className='mb-1 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Provider
+                    {t('config.provider')}
                   </label>
                   <select
                     id='provider-id'
@@ -643,7 +647,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     }}
                     className='w-full rounded border border-gray-300 dark:border-gray-600 dark:bg-gray-700 dark:text-white px-2 py-1 text-sm focus:border-blue-500 focus:outline-none focus:ring-blue-500'
                   >
-                    <option value=''>Select provider</option>
+                    <option value=''>{t('config.selectProvider')}</option>
                     {PROVIDER_OPTIONS.map((providerOption) => (
                       <option key={providerOption.id} value={providerOption.id}>
                         {providerOption.label}
@@ -657,13 +661,13 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='provider-model'
                     className='mb-1 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Model
+                    {t('config.model')}
                   </label>
                   <input
                     id='provider-model'
                     type='text'
                     value={formState.providerModel}
-                    placeholder='e.g. gpt-4o-mini'
+                    placeholder={t('config.modelPlaceholder')}
                     onChange={(event) => {
                       setField('providerModel', event.target.value);
                       resetProviderFeedback();
@@ -677,7 +681,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='provider-base-url'
                     className='mb-1 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    Base URL (optional)
+                    {t('config.baseUrlOptional')}
                   </label>
                   <input
                     id='provider-base-url'
@@ -700,13 +704,13 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                     htmlFor='provider-api-key'
                     className='mb-1 block text-sm text-gray-700 dark:text-gray-300'
                   >
-                    API key (optional for Ollama)
+                    {t('config.apiKeyOptionalOllama')}
                   </label>
                   <input
                     id='provider-api-key'
                     type='password'
                     value={formState.providerApiKey}
-                    placeholder='Enter provider API key'
+                    placeholder={t('config.apiKeyPlaceholder')}
                     onChange={(event) => {
                       setField('providerApiKey', event.target.value);
                       resetProviderFeedback();
@@ -722,7 +726,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
                   disabled={isTestingProviderConnection}
                   className='inline-flex items-center border border-transparent bg-indigo-600 px-3 py-2 text-sm font-medium text-white shadow-sm hover:bg-indigo-700 disabled:bg-gray-400 focus:outline-none'
                 >
-                  {isTestingProviderConnection ? 'Testing...' : 'Test Connection'}
+                  {isTestingProviderConnection ? t('config.testing') : t('config.testConnection')}
                 </button>
 
                 {providerTestResult && (
@@ -749,8 +753,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
           )}
 
           <p className='mt-3 text-xs text-gray-500 dark:text-gray-400'>
-            Changes are automatically saved and will be applied when switching to the Source tab.
-            Token count estimates help with optimizing context for large repositories.
+            {t('config.autoSaveHint')}
           </p>
         </div>
       </div>
@@ -761,16 +764,16 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
             onClick={() => saveConfig(formState)}
             className='inline-flex items-center border border-transparent bg-green-600 px-4 py-2 text-sm font-medium text-white shadow-sm hover:bg-green-700 focus:outline-none'
           >
-            {isSaved ? '✓ Saved' : 'Save Config'}
+            {isSaved ? t('config.savedConfig') : t('config.saveConfig')}
           </button>
         </div>
         <div className='grid grid-cols-1 md:grid-cols-2 gap-4 mb-4'>
           <div>
             <h4 className='mb-2 text-xs font-medium text-gray-700 dark:text-gray-300'>
-              Only process files with these extensions
+              {t('config.includeExtensionsTitle')}
             </h4>
             <p className='text-xs text-gray-500 dark:text-gray-400 mb-1'>
-              One extension per line (include the dot)
+              {t('config.includeExtensionsHint')}
             </p>
             <textarea
               className='h-44 w-full border border-gray-300 dark:border-gray-600 dark:bg-gray-700 dark:text-white p-2 font-mono text-sm shadow-sm focus:border-blue-500 focus:outline-none focus:ring-blue-500'
@@ -785,10 +788,10 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
           </div>
           <div>
             <h4 className='mb-2 text-xs font-medium text-gray-700 dark:text-gray-300'>
-              Exclude Patterns
+              {t('config.excludePatternsTitle')}
             </h4>
             <p className='text-xs text-gray-500 dark:text-gray-400 mb-1'>
-              One pattern per line (using glob pattern)
+              {t('config.excludePatternsHint')}
             </p>
             <textarea
               className='h-44 w-full border border-gray-300 dark:border-gray-600 dark:bg-gray-700 dark:text-white p-2 font-mono text-sm shadow-sm focus:border-blue-500 focus:outline-none focus:ring-blue-500'
@@ -804,7 +807,7 @@ const ConfigTab = ({ configContent, onConfigChange }: ConfigTabProps) => {
       </div>
 
       <div className='mt-4 text-xs text-gray-500 dark:text-gray-400'>
-        <p>Configure which file types to include and patterns to exclude in the analysis.</p>
+        <p>{t('config.configSummary')}</p>
       </div>
     </div>
   );
diff --git a/src/renderer/components/DarkModeToggle.tsx b/src/renderer/components/DarkModeToggle.tsx
index e83b8d7..e6720eb 100644
--- a/src/renderer/components/DarkModeToggle.tsx
+++ b/src/renderer/components/DarkModeToggle.tsx
@@ -1,16 +1,18 @@
 import React from 'react';
+import { useTranslation } from 'react-i18next';
 
 import { useDarkMode } from '../context/DarkModeContext';
 
 const DarkModeToggle = () => {
+  const { t } = useTranslation();
   const { darkMode, toggleDarkMode } = useDarkMode();
 
   return (
     <button
       onClick={toggleDarkMode}
       className='p-2 rounded-md hover:bg-gray-200 dark:hover:bg-gray-700'
-      title={darkMode ? 'Switch to Light Mode' : 'Switch to Dark Mode'}
-      aria-label={darkMode ? 'Switch to Light Mode' : 'Switch to Dark Mode'}
+      title={darkMode ? t('darkMode.switchToLight') : t('darkMode.switchToDark')}
+      aria-label={darkMode ? t('darkMode.switchToLight') : t('darkMode.switchToDark')}
     >
       {darkMode ? (
         // Sun icon for light mode
diff --git a/src/renderer/components/FileTree.tsx b/src/renderer/components/FileTree.tsx
index e8137a2..7d515e9 100755
--- a/src/renderer/components/FileTree.tsx
+++ b/src/renderer/components/FileTree.tsx
@@ -1,4 +1,5 @@
 import React, { useMemo, useState } from 'react';
+import { useTranslation } from 'react-i18next';
 
 import type { DirectoryTreeItem, SelectionHandler } from '../../types/ipc';
 
@@ -40,6 +41,7 @@ const FileTreeItemComponent = ({
   onFileSelect,
   onFolderSelect,
 }: FileTreeItemProps) => {
+  const { t } = useTranslation();
   const isFile = item.type === 'file';
   const isFolder = item.type === 'directory';
   const checkboxIsSelected = getSelectionStatus(item, selectedFiles, selectedFolders);
@@ -115,7 +117,7 @@ const FileTreeItemComponent = ({
             type='button'
             className='mr-1 size-5 shrink-0 rounded text-gray-500 dark:text-gray-400 hover:bg-gray-200 dark:hover:bg-gray-700 focus:outline-none'
             onClick={handleToggle}
-            aria-label={isOpen ? 'Collapse folder' : 'Expand folder'}
+            aria-label={isOpen ? t('fileTree.collapseFolder') : t('fileTree.expandFolder')}
           >
             <span className='block text-center'>{isOpen ? '▼' : '▶'}</span>
           </button>
@@ -148,7 +150,11 @@ const FileTreeItemComponent = ({
                   }
                 }}
                 aria-expanded={isOpen}
-                aria-label={`${isOpen ? 'Collapse' : 'Expand'} folder ${item.name}`}
+                aria-label={
+                  isOpen
+                    ? t('fileTree.collapseFolderWithName', { name: item.name })
+                    : t('fileTree.expandFolderWithName', { name: item.name })
+                }
               >
                 <span className='shrink-0 text-yellow-500' aria-hidden='true'>
                   {isOpen ? '📂' : '📁'}
@@ -243,6 +249,7 @@ const FileTreeComponent = ({
   onFolderSelect,
   onBatchSelect,
 }: FileTreeProps) => {
+  const { t } = useTranslation();
   const totalFiles = useMemo(() => countTotalFiles(items), [items]);
 
   const selectAllChecked = useMemo(() => {
@@ -292,11 +299,11 @@ const FileTreeComponent = ({
             htmlFor='select-all-checkbox'
             className='cursor-pointer select-none text-sm font-medium text-gray-700 dark:text-gray-300'
           >
-            Select All
+            {t('fileTree.selectAll')}
           </label>
         </div>
         <span className='text-xs font-medium text-gray-500 dark:text-gray-400'>
-          <span className='font-medium'>{selectedFiles.size}</span> of {totalFiles} files selected
+          {t('fileTree.selectedCount', { selected: selectedFiles.size, total: totalFiles })}
         </span>
       </div>
 
@@ -317,8 +324,8 @@ const FileTreeComponent = ({
                 d='M5 19a2 2 0 01-2-2V7a2 2 0 012-2h4l2 2h4a2 2 0 012 2v1M5 19h14a2 2 0 002-2v-5a2 2 0 00-2-2H9a2 2 0 00-2 2v5a2 2 0 01-2 2z'
               ></path>
             </svg>
-            <p>No files to display</p>
-            <p className='mt-2 text-sm'>Select a directory to view files</p>
+            <p>{t('fileTree.emptyTitle')}</p>
+            <p className='mt-2 text-sm'>{t('fileTree.emptyHint')}</p>
           </div>
         ) : (
           items.map((item) => (
diff --git a/src/renderer/components/LanguageSelector.tsx b/src/renderer/components/LanguageSelector.tsx
new file mode 100644
index 0000000..236f9c9
--- /dev/null
+++ b/src/renderer/components/LanguageSelector.tsx
@@ -0,0 +1,42 @@
+import React from 'react';
+import { useTranslation } from 'react-i18next';
+
+import { persistLocale, SUPPORTED_LOCALES, type SupportedLocale } from '../i18n/settings';
+
+const LanguageSelector = () => {
+  const { i18n, t } = useTranslation();
+
+  const resolvedLanguage = (i18n.resolvedLanguage || i18n.language || 'en').split('-')[0];
+  const selectedLocale = (SUPPORTED_LOCALES.includes(resolvedLanguage as SupportedLocale)
+    ? resolvedLanguage
+    : 'en') as SupportedLocale;
+
+  const handleLanguageChange = async (event: React.ChangeEvent<HTMLSelectElement>) => {
+    const nextLocale = event.target.value as SupportedLocale;
+    await i18n.changeLanguage(nextLocale);
+    persistLocale(nextLocale);
+  };
+
+  return (
+    <div className='flex items-center'>
+      <label htmlFor='language-selector' className='mr-2 text-sm text-gray-600 dark:text-gray-300'>
+        {t('languageSelector.label')}
+      </label>
+      <select
+        id='language-selector'
+        data-testid='language-selector'
+        value={selectedLocale}
+        onChange={handleLanguageChange}
+        className='rounded border border-gray-300 dark:border-gray-700 bg-white dark:bg-gray-700 px-2 py-1 text-sm text-gray-700 dark:text-gray-200 focus:border-blue-500 focus:outline-none focus:ring-1 focus:ring-blue-500'
+      >
+        {SUPPORTED_LOCALES.map((locale) => (
+          <option key={locale} value={locale}>
+            {t(`languageSelector.${locale}`)}
+          </option>
+        ))}
+      </select>
+    </div>
+  );
+};
+
+export default LanguageSelector;
diff --git a/src/renderer/components/ProcessedTab.tsx b/src/renderer/components/ProcessedTab.tsx
index cf0bfb0..69668c9 100755
--- a/src/renderer/components/ProcessedTab.tsx
+++ b/src/renderer/components/ProcessedTab.tsx
@@ -1,4 +1,5 @@
 import React, { useState } from 'react';
+import { useTranslation } from 'react-i18next';
 
 import Spinner from './icons/Spinner';
 
@@ -11,6 +12,7 @@ type ProcessedTabProps = {
 };
 
 const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps) => {
+  const { t } = useTranslation();
   const [isSaving, setIsSaving] = useState(false);
   const [isCopied, setIsCopied] = useState(false);
   const [isRefreshing, setIsRefreshing] = useState(false);
@@ -68,12 +70,12 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                 onClick={handleRefresh}
                 className='inline-flex items-center border border-transparent bg-green-600 px-5 py-2 text-sm font-medium text-white shadow-sm hover:bg-green-700 focus:outline-none focus:ring-2 focus:ring-green-500 focus:ring-offset-2'
                 disabled={isRefreshing}
-                title='Reload selected files and regenerate output with latest content'
+                title={t('processed.refreshCodeTitle')}
               >
                 {isRefreshing ? (
                   <>
                     <Spinner className='-ml-1 mr-2 h-4 w-4' />
-                    Reprocessing...
+                    {t('processed.reprocessing')}
                   </>
                 ) : (
                   <>
@@ -91,7 +93,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                         d='M4 4v5h.582m15.356 2A8.001 8.001 0 004.582 9m0 0H9m11 11v-5h-.581m0 0a8.003 8.003 0 01-15.357-2m15.357 2H15'
                       />
                     </svg>
-                    Refresh Code
+                    {t('processed.refreshCode')}
                   </>
                 )}
               </button>
@@ -100,7 +102,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
             {/* Stats in the center */}
             <div className='flex items-center'>
               <div className='flex items-center'>
-                <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>Files</span>
+                <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>{t('common.files')}</span>
                 <span className='text-lg font-bold text-blue-600 dark:text-blue-400'>
                   {processedResult.processedFiles}
                 </span>
@@ -109,7 +111,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
               <div className='text-gray-400 mx-3'>|</div>
 
               <div className='flex items-center'>
-                <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>Tokens</span>
+                <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>{t('common.tokens')}</span>
                 <span className='text-lg font-bold text-green-600 dark:text-green-400'>
                   {processedResult.totalTokens.toLocaleString()}
                 </span>
@@ -119,7 +121,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                 <>
                   <div className='text-gray-400 mx-3'>|</div>
                   <div className='flex items-center'>
-                    <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>Skipped</span>
+                    <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>{t('processed.skipped')}</span>
                     <span className='text-lg font-bold text-amber-600 dark:text-amber-400'>
                       {processedResult.skippedFiles}
                     </span>
@@ -134,7 +136,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                 className='inline-flex items-center border border-transparent bg-green-600 px-4 py-2 text-sm font-medium text-white shadow-sm hover:bg-green-700 focus:outline-none'
               >
                 {isCopied ? (
-                  '✓ Copied'
+                  t('processed.copied')
                 ) : (
                   <>
                     <svg
@@ -146,7 +148,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                       <path d='M4 1.5H3a2 2 0 0 0-2 2V14a2 2 0 0 0 2 2h10a2 2 0 0 0 2-2V3.5a2 2 0 0 0-2-2h-1v1h1a1 1 0 0 1 1 1V14a1 1 0 0 1-1 1H3a1 1 0 0 1-1-1V3.5a1 1 0 0 1 1-1h1v-1z' />
                       <path d='M9.5 1a.5.5 0 0 1 .5.5v1a.5.5 0 0 1-.5.5h-3a.5.5 0 0 1-.5-.5v-1a.5.5 0 0 1 .5-.5h3zm-3-1A1.5 1.5 0 0 0 5 1.5v1A1.5 1.5 0 0 0 6.5 4h3A1.5 1.5 0 0 0 11 2.5v-1A1.5 1.5 0 0 0 9.5 0h-3z' />
                     </svg>
-                    Copy Content
+                    {t('processed.copyContent')}
                   </>
                 )}
               </button>
@@ -157,7 +159,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                 } focus:outline-none`}
               >
                 {isSaving ? (
-                  '✓ Saving...'
+                  t('processed.saving')
                 ) : (
                   <>
                     <svg
@@ -174,7 +176,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                         d='M8 7H5a2 2 0 00-2 2v9a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-3m-1 4l-3 3m0 0l-3-3m3 3V4'
                       />
                     </svg>
-                    Save to File
+                    {t('processed.saveToFile')}
                   </>
                 )}
               </button>
@@ -187,10 +189,10 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                 htmlFor='processed-content'
                 className='block text-sm font-medium text-gray-700 dark:text-gray-300'
               >
-                Processed Content
+                {t('processed.processedContent')}
               </label>
               <div className='text-xs text-gray-500 dark:text-gray-400'>
-                Content is ready to be saved
+                {t('processed.contentReady')}
               </div>
             </div>
             <div
@@ -206,7 +208,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
           {/* Files by Token Count section added to ProcessedTab */}
           <div className='mt-6'>
             <h3 className='mb-2 text-base font-medium text-gray-900 dark:text-gray-100'>
-              Files by Token Count
+              {t('processed.filesByTokenCount')}
             </h3>
             <div className='rounded-md border border-gray-200 dark:border-gray-700 shadow-sm'>
               <div className='h-[35vh] min-h-48 max-h-[45vh] overflow-y-auto overflow-x-hidden'>
@@ -214,10 +216,10 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                   <thead className='sticky top-0 bg-gray-50 dark:bg-gray-800'>
                     <tr>
                       <th className='px-3 py-1 text-left text-xs font-medium uppercase tracking-wider text-gray-500 dark:text-gray-400'>
-                        File Path
+                        {t('processed.filePath')}
                       </th>
                       <th className='px-3 py-1 text-right text-xs font-medium uppercase tracking-wider text-gray-500 dark:text-gray-400'>
-                        Tokens
+                        {t('common.tokens')}
                       </th>
                     </tr>
                   </thead>
@@ -242,7 +244,7 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
                           colSpan={2}
                           className='px-3 py-4 text-center text-sm text-gray-500 dark:text-gray-400'
                         >
-                          No file data available
+                          {t('processed.noFileData')}
                         </td>
                       </tr>
                     )}
@@ -269,10 +271,10 @@ const ProcessedTab = ({ processedResult, onSave, onRefresh }: ProcessedTabProps)
             ></path>
           </svg>
           <p className='mb-2 font-medium text-gray-500 dark:text-gray-400'>
-            No processed content yet
+            {t('processed.noProcessedContent')}
           </p>
           <p className='px-6 text-center text-sm text-gray-400 dark:text-gray-500'>
-            Go to the Source tab to select files, then analyze and process them.
+            {t('processed.noProcessedContentHint')}
           </p>
         </div>
       )}
diff --git a/src/renderer/components/SourceTab.tsx b/src/renderer/components/SourceTab.tsx
index 2aad483..51603d4 100755
--- a/src/renderer/components/SourceTab.tsx
+++ b/src/renderer/components/SourceTab.tsx
@@ -1,4 +1,5 @@
 import React, { useEffect, useRef, useState } from 'react';
+import { useTranslation } from 'react-i18next';
 import yaml from 'yaml';
 
 import FileTree from './FileTree';
@@ -73,6 +74,7 @@ const SourceTab = ({
   onAnalyze,
   onRefreshTree,
 }: SourceTabProps) => {
+  const { t } = useTranslation();
   const [isAnalyzing, setIsAnalyzing] = useState(false);
   const [showTokenCount, setShowTokenCount] = useState(true);
   const [totalTokens, setTotalTokens] = useState(0);
@@ -204,7 +206,7 @@ const SourceTab = ({
       return (
         <>
           <Spinner className='-ml-1 mr-2 h-4 w-4 text-white' data-testid='process-selected-files-spinner' />
-          Processing...
+          {t('source.processingSelectedFiles')}
         </>
       );
     }
@@ -213,7 +215,7 @@ const SourceTab = ({
       return (
         <>
           <Spinner className='-ml-1 mr-2 h-4 w-4 text-white' data-testid='process-selected-files-spinner' />
-          Selecting files...
+          {t('source.selectingFiles')}
         </>
       );
     }
@@ -234,7 +236,7 @@ const SourceTab = ({
             d='M13 10V3L4 14h7v7l9-11h-7z'
           />
         </svg>
-        Process Selected Files
+        {t('source.processSelectedFiles')}
       </>
     );
   };
@@ -248,7 +250,7 @@ const SourceTab = ({
             htmlFor='file-folder-selection'
             className='block text-sm font-medium text-gray-700 dark:text-gray-300'
           >
-            Select Files and Folders
+            {t('source.selectFilesAndFolders')}
           </label>
         </div>
 
@@ -284,7 +286,7 @@ const SourceTab = ({
             d='M5 19a2 2 0 01-2-2V7a2 2 0 012-2h4l2 2h4a2 2 0 012 2v1M5 19h14a2 2 0 002-2v-5a2 2 0 00-2-2H9a2 2 0 00-2 2v5a2 2 0 01-2 2z'
           ></path>
         </svg>
-        <p className='mt-2 text-gray-500 dark:text-gray-400'>Loading directory content...</p>
+        <p className='mt-2 text-gray-500 dark:text-gray-400'>{t('source.loadingDirectory')}</p>
       </div>
     );
   }
@@ -298,9 +300,9 @@ const SourceTab = ({
             className='grow border border-gray-300 dark:border-gray-700 dark:bg-gray-700 dark:text-white px-3 py-2 shadow-sm focus:border-blue-500 focus:outline-none focus:ring-blue-500 cursor-pointer'
             value={rootPath}
             readOnly
-            placeholder='Select a root folder'
+            placeholder={t('source.selectRootFolderPlaceholder')}
             onClick={handleDirectorySelect}
-            title='Click to browse for a directory'
+            title={t('source.browseDirectoryTitle')}
           />
           <button
             onClick={handleDirectorySelect}
@@ -320,7 +322,7 @@ const SourceTab = ({
                 d='M3 7v10a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-6l-2-2H5a2 2 0 00-2 2z'
               />
             </svg>
-            Change Folder
+            {t('source.changeFolder')}
           </button>
         </div>
       </div>
@@ -339,7 +341,7 @@ const SourceTab = ({
                 await onRefreshTree();
               }}
               className='inline-flex items-center border border-transparent bg-green-600 px-4 py-2 text-sm font-medium text-white shadow-sm hover:bg-green-700 focus:outline-none focus:ring-2 focus:ring-green-500 focus:ring-offset-2 dark:focus:ring-offset-gray-800'
-              title='Refresh the file list'
+              title={t('source.refreshFileListTitle')}
             >
               <svg
                 className='size-4 mr-1'
@@ -355,7 +357,7 @@ const SourceTab = ({
                   d='M4 4v5h.582m15.356 2A8.001 8.001 0 004.582 9m0 0H9m11 11v-5h-.581m0 0a8.003 8.003 0 01-15.357-2m15.357 2H15'
                 />
               </svg>
-              Refresh file list
+              {t('source.refreshFileList')}
             </button>
 
             <button
@@ -369,7 +371,7 @@ const SourceTab = ({
                 setIsCalculating(false);
               }}
               className='inline-flex items-center border border-transparent bg-gray-600 px-4 py-2 text-sm font-medium text-white shadow-sm hover:bg-gray-700 focus:outline-none focus:ring-2 focus:ring-gray-500 focus:ring-offset-2 dark:focus:ring-offset-gray-800'
-              title='Clear all selected files'
+              title={t('source.clearSelectionTitle')}
             >
               <svg
                 className='size-4 mr-1'
@@ -385,14 +387,14 @@ const SourceTab = ({
                   d='M6 18L18 6M6 6l12 12'
                 />
               </svg>
-              Clear selection
+              {t('source.clearSelection')}
             </button>
           </div>
         )}
 
         <div className='ml-auto flex items-center space-x-4'>
           <div className='flex items-center'>
-            <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>Files</span>
+            <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>{t('common.files')}</span>
             <span className='text-lg font-bold text-blue-600 dark:text-blue-400'>
               {selectedFiles.size}
             </span>
@@ -402,7 +404,7 @@ const SourceTab = ({
             <>
               <div className='text-gray-400 mx-1'>|</div>
               <div className='flex items-center'>
-                <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>Tokens</span>
+                <span className='text-sm text-gray-500 dark:text-gray-400 mr-2'>{t('common.tokens')}</span>
                 <span className='text-lg font-bold text-green-600 dark:text-green-400'>
                   {totalTokens.toLocaleString()}
                 </span>
@@ -436,7 +438,7 @@ const SourceTab = ({
         <div className='mt-4 p-4 bg-blue-50 rounded-md border border-blue-100 dark:border-blue-800 dark:bg-blue-900/30'>
           <div className='flex items-center justify-center text-blue-700 dark:text-blue-300'>
             <Spinner className='-ml-1 mr-3 h-5 w-5' />
-            Analyzing selected files, please wait...
+            {t('source.analyzingWait')}
           </div>
         </div>
       )}
diff --git a/src/renderer/components/TabBar.tsx b/src/renderer/components/TabBar.tsx
index cd7d085..0ba1c18 100755
--- a/src/renderer/components/TabBar.tsx
+++ b/src/renderer/components/TabBar.tsx
@@ -1,4 +1,5 @@
 import React from 'react';
+import { useTranslation } from 'react-i18next';
 
 import type { TabId } from '../../types/ipc';
 
@@ -8,10 +9,11 @@ type TabBarProps = {
 };
 
 const TabBar = ({ activeTab, onTabChange }: TabBarProps) => {
+  const { t } = useTranslation();
   const tabs = [
-    { id: 'config', label: 'Start' },
-    { id: 'source', label: 'Select Files' },
-    { id: 'processed', label: 'Processed Output' },
+    { id: 'config', labelKey: 'tabs.config' },
+    { id: 'source', labelKey: 'tabs.source' },
+    { id: 'processed', labelKey: 'tabs.processed' },
   ] as const;
 
   return (
@@ -30,8 +32,9 @@ const TabBar = ({ activeTab, onTabChange }: TabBarProps) => {
           }`}
           onClick={() => onTabChange(tab.id)}
           data-tab={tab.id}
+          data-testid={`tab-${tab.id}`}
         >
-          {tab.label}
+          {t(tab.labelKey)}
         </button>
       ))}
     </div>
diff --git a/src/renderer/context/AppContext.tsx b/src/renderer/context/AppContext.tsx
index 1919fea..b07e13c 100644
--- a/src/renderer/context/AppContext.tsx
+++ b/src/renderer/context/AppContext.tsx
@@ -2,6 +2,7 @@ import React, { createContext, useCallback, useContext, useEffect, useMemo, useR
 import yaml from 'yaml';
 
 import { normalizeExportFormat } from '../../utils/export-format';
+import i18n from '../i18n';
 
 import type {
   AnalyzeRepositoryResult,
@@ -437,7 +438,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
   const handleAnalyze = useCallback(async (): Promise<AnalyzeRepositoryResult | undefined> => {
     const selectedFilesArray = [...selectedFiles];
     if (!rootPath || selectedFilesArray.length === 0) {
-      showError('Please select a root directory and at least one file.');
+      showError(i18n.t('errors.selectRootAndFiles'));
       return undefined;
     }
 
@@ -452,14 +453,12 @@ export const AppProvider = ({ children }: AppProviderProps) => {
       });
 
       if (validFiles.length === 0) {
-        showError(
-          'No valid files selected for analysis. Please select files within the current directory.'
-        );
+        showError(i18n.t('errors.noValidFiles'));
         return undefined;
       }
 
       if (!appWindow.electronAPI?.analyzeRepository || !appWindow.electronAPI?.processRepository) {
-        throw new Error('Electron API is not available.');
+        throw new Error(i18n.t('errors.electronApiUnavailable'));
       }
 
       const currentAnalysisResult = await appWindow.electronAPI.analyzeRepository({
@@ -504,7 +503,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     } catch (error) {
       const processedError = ensureError(error);
       console.error('Error processing repository:', processedError);
-      showError('An error occurred while processing the repository. Check the console for details.');
+      showError(i18n.t('errors.processingFailed'));
       throw processedError;
     }
   }, [selectedFiles, rootPath, configContent, appWindow, showError]);
@@ -513,14 +512,12 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     try {
       const selectedFilesArray = [...selectedFiles];
       if (!rootPath || selectedFilesArray.length === 0) {
-        showError(
-          'No files are selected for processing. Please go to the Source tab and select files.'
-        );
+        showError(i18n.t('errors.noFilesSelectedForProcessing'));
         return null;
       }
 
       if (!appWindow.electronAPI?.analyzeRepository || !appWindow.electronAPI?.processRepository) {
-        throw new Error('Electron API is not available.');
+        throw new Error(i18n.t('errors.electronApiUnavailable'));
       }
 
       const currentReanalysisResult = await appWindow.electronAPI.analyzeRepository({
@@ -561,14 +558,14 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     } catch (error) {
       const processedError = ensureError(error);
       console.error('Error refreshing processed content:', processedError);
-      showError('An error occurred while refreshing content. Check the console for details.');
+      showError(i18n.t('errors.refreshFailed'));
       throw processedError;
     }
   }, [selectedFiles, rootPath, configContent, appWindow, processingOptions, showError]);
 
   const handleSaveOutput = useCallback(async () => {
     if (!processedResult) {
-      showError('No processed content to save.');
+      showError(i18n.t('errors.noProcessedContentToSave'));
       return;
     }
 
@@ -581,7 +578,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     } catch (error) {
       const processedError = ensureError(error);
       console.error('Error saving file:', processedError);
-      showError('An error occurred while saving the file. Check the console for details.');
+      showError(i18n.t('errors.saveFailed'));
     }
   }, [processedResult, appWindow, rootPath, showError]);
 
diff --git a/src/renderer/i18n/index.ts b/src/renderer/i18n/index.ts
new file mode 100644
index 0000000..d77fa61
--- /dev/null
+++ b/src/renderer/i18n/index.ts
@@ -0,0 +1,33 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+
+import deCommon from './locales/de/common.json';
+import enCommon from './locales/en/common.json';
+import esCommon from './locales/es/common.json';
+import frCommon from './locales/fr/common.json';
+import { DEFAULT_LOCALE, getInitialLocale, SUPPORTED_LOCALES } from './settings';
+
+const resources = {
+  en: { common: enCommon },
+  es: { common: esCommon },
+  fr: { common: frCommon },
+  de: { common: deCommon },
+};
+
+if (!i18n.isInitialized) {
+  i18n.use(initReactI18next).init({
+    resources,
+    lng: getInitialLocale(),
+    supportedLngs: [...SUPPORTED_LOCALES],
+    fallbackLng: DEFAULT_LOCALE,
+    defaultNS: 'common',
+    ns: ['common'],
+    initImmediate: false,
+    interpolation: {
+      escapeValue: false,
+    },
+    returnNull: false,
+  });
+}
+
+export default i18n;
diff --git a/src/renderer/i18n/locales/de/common.json b/src/renderer/i18n/locales/de/common.json
new file mode 100644
index 0000000..f543710
--- /dev/null
+++ b/src/renderer/i18n/locales/de/common.json
@@ -0,0 +1,127 @@
+{
+  "app": {
+    "name": "AI Code Fusion",
+    "github": "Auf GitHub ansehen",
+    "dismissError": "Fehler schließen"
+  },
+  "languageSelector": {
+    "label": "Sprache",
+    "en": "Englisch",
+    "es": "Spanisch",
+    "fr": "Französisch",
+    "de": "Deutsch"
+  },
+  "darkMode": {
+    "switchToLight": "Zum hellen Modus wechseln",
+    "switchToDark": "Zum dunklen Modus wechseln"
+  },
+  "tabs": {
+    "config": "Start",
+    "source": "Dateien auswählen",
+    "processed": "Verarbeitete Ausgabe"
+  },
+  "common": {
+    "files": "Dateien",
+    "tokens": "Tokens"
+  },
+  "config": {
+    "selectRootFolderPlaceholder": "Stammordner auswählen",
+    "browseDirectoryTitle": "Klicken, um ein Verzeichnis auszuwählen",
+    "selectFolder": "Ordner auswählen",
+    "fileFilteringTitle": "Dateifilter",
+    "filterByExtensions": "Nach Dateiendungen filtern",
+    "useExcludePatterns": "Ausschlussmuster verwenden",
+    "applyGitignoreRules": ".gitignore-Regeln anwenden",
+    "scanSecrets": "Inhalte auf Geheimnisse prüfen",
+    "excludeSuspiciousFiles": "Verdächtige Dateien ausschließen",
+    "outputFormattingTitle": "Ausgabeformatierung",
+    "includeFileTree": "Dateibaum in Ausgabe einfügen",
+    "displayTokenCounts": "Token-Anzahlen anzeigen",
+    "exportFormat": "Exportformat",
+    "exportFormatMarkdown": "Markdown",
+    "exportFormatXml": "XML",
+    "providerSetupTitle": "Einrichtungsassistent für Anbieter",
+    "providerSetupDescription": "Konfiguriere einen Modellanbieter und führe vor dem Speichern einen Verbindungstest aus.",
+    "provider": "Anbieter",
+    "selectProvider": "Anbieter auswählen",
+    "model": "Modell",
+    "modelPlaceholder": "z. B. gpt-4o-mini",
+    "baseUrlOptional": "Basis-URL (optional)",
+    "apiKeyOptionalOllama": "API-Schlüssel (optional für Ollama)",
+    "apiKeyPlaceholder": "API-Schlüssel des Anbieters eingeben",
+    "testing": "Teste...",
+    "testConnection": "Verbindung testen",
+    "autoSaveHint": "Änderungen werden automatisch gespeichert und beim Wechsel zum Quell-Tab angewendet. Token-Schätzungen helfen, den Kontext für große Repositories zu optimieren.",
+    "saveConfig": "Konfiguration speichern",
+    "savedConfig": "✓ Gespeichert",
+    "includeExtensionsTitle": "Nur Dateien mit diesen Endungen verarbeiten",
+    "includeExtensionsHint": "Eine Endung pro Zeile (mit Punkt)",
+    "excludePatternsTitle": "Ausschlussmuster",
+    "excludePatternsHint": "Ein Muster pro Zeile (Glob-Syntax)",
+    "configSummary": "Konfiguriere, welche Dateitypen einbezogen und welche Muster in der Analyse ausgeschlossen werden.",
+    "providerTestDisabled": "Der Anbieter-Verbindungstest ist außerhalb des Entwicklungsmodus deaktiviert.",
+    "providerFixBeforeTesting": "Korrigiere die Anbieter-Einstellungen, bevor du die Verbindung testest.",
+    "providerUnavailable": "Der Anbieter-Verbindungstest ist in diesem Build nicht verfügbar.",
+    "providerFixBeforeSaving": "Korrigiere die Anbieter-Einstellungen vor dem Speichern.",
+    "connectionTestFailed": "Verbindungstest fehlgeschlagen: {{message}}",
+    "validation": {
+      "selectProvider": "Wähle einen Anbieter aus.",
+      "modelRequired": "Modell ist erforderlich.",
+      "apiKeyRequired": "Für diesen Anbieter ist ein API-Schlüssel erforderlich.",
+      "baseUrlProtocol": "Die Basis-URL muss http oder https verwenden.",
+      "baseUrlValid": "Die Basis-URL muss gültig sein."
+    }
+  },
+  "source": {
+    "processSelectedFiles": "Ausgewählte Dateien verarbeiten",
+    "processingSelectedFiles": "Verarbeite...",
+    "selectingFiles": "Dateien werden ausgewählt...",
+    "selectFilesAndFolders": "Dateien und Ordner auswählen",
+    "loadingDirectory": "Verzeichnisinhalt wird geladen...",
+    "selectRootFolderPlaceholder": "Stammordner auswählen",
+    "browseDirectoryTitle": "Klicken, um ein Verzeichnis auszuwählen",
+    "changeFolder": "Ordner ändern",
+    "refreshFileList": "Dateiliste aktualisieren",
+    "refreshFileListTitle": "Dateiliste aktualisieren",
+    "clearSelection": "Auswahl löschen",
+    "clearSelectionTitle": "Alle ausgewählten Dateien löschen",
+    "analyzingWait": "Ausgewählte Dateien werden analysiert, bitte warten..."
+  },
+  "processed": {
+    "refreshCode": "Code aktualisieren",
+    "refreshCodeTitle": "Ausgewählte Dateien neu laden und Ausgabe mit aktuellem Inhalt neu erzeugen",
+    "reprocessing": "Erneut verarbeiten...",
+    "skipped": "Übersprungen",
+    "copied": "✓ Kopiert",
+    "copyContent": "Inhalt kopieren",
+    "saving": "✓ Speichere...",
+    "saveToFile": "In Datei speichern",
+    "processedContent": "Verarbeiteter Inhalt",
+    "contentReady": "Inhalt ist zum Speichern bereit",
+    "filesByTokenCount": "Dateien nach Token-Anzahl",
+    "filePath": "Dateipfad",
+    "noFileData": "Keine Dateidaten verfügbar",
+    "noProcessedContent": "Noch kein verarbeiteter Inhalt",
+    "noProcessedContentHint": "Wechsle zum Quell-Tab, wähle Dateien aus und analysiere/verarbeite sie dann."
+  },
+  "fileTree": {
+    "collapseFolder": "Ordner einklappen",
+    "expandFolder": "Ordner ausklappen",
+    "collapseFolderWithName": "Ordner {{name}} einklappen",
+    "expandFolderWithName": "Ordner {{name}} ausklappen",
+    "selectAll": "Alle auswählen",
+    "selectedCount": "{{selected}} von {{total}} Dateien ausgewählt",
+    "emptyTitle": "Keine Dateien zum Anzeigen",
+    "emptyHint": "Wähle ein Verzeichnis, um Dateien anzuzeigen"
+  },
+  "errors": {
+    "selectRootAndFiles": "Bitte wähle ein Stammverzeichnis und mindestens eine Datei aus.",
+    "noValidFiles": "Keine gültigen Dateien zur Analyse ausgewählt. Bitte wähle Dateien innerhalb des aktuellen Verzeichnisses aus.",
+    "electronApiUnavailable": "Electron-API ist nicht verfügbar.",
+    "processingFailed": "Beim Verarbeiten des Repositories ist ein Fehler aufgetreten. Details in der Konsole.",
+    "noFilesSelectedForProcessing": "Es sind keine Dateien zur Verarbeitung ausgewählt. Wechsle zum Quell-Tab und wähle Dateien aus.",
+    "refreshFailed": "Beim Aktualisieren des Inhalts ist ein Fehler aufgetreten. Details in der Konsole.",
+    "noProcessedContentToSave": "Kein verarbeiteter Inhalt zum Speichern vorhanden.",
+    "saveFailed": "Beim Speichern der Datei ist ein Fehler aufgetreten. Details in der Konsole."
+  }
+}
diff --git a/src/renderer/i18n/locales/en/common.json b/src/renderer/i18n/locales/en/common.json
new file mode 100644
index 0000000..13af387
--- /dev/null
+++ b/src/renderer/i18n/locales/en/common.json
@@ -0,0 +1,127 @@
+{
+  "app": {
+    "name": "AI Code Fusion",
+    "github": "View on GitHub",
+    "dismissError": "Dismiss error"
+  },
+  "languageSelector": {
+    "label": "Language",
+    "en": "English",
+    "es": "Spanish",
+    "fr": "French",
+    "de": "German"
+  },
+  "darkMode": {
+    "switchToLight": "Switch to Light Mode",
+    "switchToDark": "Switch to Dark Mode"
+  },
+  "tabs": {
+    "config": "Start",
+    "source": "Select Files",
+    "processed": "Processed Output"
+  },
+  "common": {
+    "files": "Files",
+    "tokens": "Tokens"
+  },
+  "config": {
+    "selectRootFolderPlaceholder": "Select a root folder",
+    "browseDirectoryTitle": "Click to browse for a directory",
+    "selectFolder": "Select Folder",
+    "fileFilteringTitle": "File Filtering",
+    "filterByExtensions": "Filter by file extensions",
+    "useExcludePatterns": "Use exclude patterns",
+    "applyGitignoreRules": "Apply .gitignore rules",
+    "scanSecrets": "Scan content for secrets",
+    "excludeSuspiciousFiles": "Exclude suspicious files",
+    "outputFormattingTitle": "Output Formatting",
+    "includeFileTree": "Include file tree in output",
+    "displayTokenCounts": "Display token counts",
+    "exportFormat": "Export format",
+    "exportFormatMarkdown": "Markdown",
+    "exportFormatXml": "XML",
+    "providerSetupTitle": "Provider Setup Assistant",
+    "providerSetupDescription": "Configure a model provider and run a connection test before saving.",
+    "provider": "Provider",
+    "selectProvider": "Select provider",
+    "model": "Model",
+    "modelPlaceholder": "e.g. gpt-4o-mini",
+    "baseUrlOptional": "Base URL (optional)",
+    "apiKeyOptionalOllama": "API key (optional for Ollama)",
+    "apiKeyPlaceholder": "Enter provider API key",
+    "testing": "Testing...",
+    "testConnection": "Test Connection",
+    "autoSaveHint": "Changes are automatically saved and will be applied when switching to the Source tab. Token count estimates help with optimizing context for large repositories.",
+    "saveConfig": "Save Config",
+    "savedConfig": "✓ Saved",
+    "includeExtensionsTitle": "Only process files with these extensions",
+    "includeExtensionsHint": "One extension per line (include the dot)",
+    "excludePatternsTitle": "Exclude Patterns",
+    "excludePatternsHint": "One pattern per line (using glob pattern)",
+    "configSummary": "Configure which file types to include and patterns to exclude in the analysis.",
+    "providerTestDisabled": "Provider connection testing is disabled outside dev mode.",
+    "providerFixBeforeTesting": "Fix provider settings before testing the connection.",
+    "providerUnavailable": "Provider connection testing is unavailable in this build.",
+    "providerFixBeforeSaving": "Fix provider settings before saving.",
+    "connectionTestFailed": "Connection test failed: {{message}}",
+    "validation": {
+      "selectProvider": "Select a provider.",
+      "modelRequired": "Model is required.",
+      "apiKeyRequired": "API key is required for this provider.",
+      "baseUrlProtocol": "Base URL must use http or https.",
+      "baseUrlValid": "Base URL must be a valid URL."
+    }
+  },
+  "source": {
+    "processSelectedFiles": "Process Selected Files",
+    "processingSelectedFiles": "Processing...",
+    "selectingFiles": "Selecting files...",
+    "selectFilesAndFolders": "Select Files and Folders",
+    "loadingDirectory": "Loading directory content...",
+    "selectRootFolderPlaceholder": "Select a root folder",
+    "browseDirectoryTitle": "Click to browse for a directory",
+    "changeFolder": "Change Folder",
+    "refreshFileList": "Refresh file list",
+    "refreshFileListTitle": "Refresh the file list",
+    "clearSelection": "Clear selection",
+    "clearSelectionTitle": "Clear all selected files",
+    "analyzingWait": "Analyzing selected files, please wait..."
+  },
+  "processed": {
+    "refreshCode": "Refresh Code",
+    "refreshCodeTitle": "Reload selected files and regenerate output with latest content",
+    "reprocessing": "Reprocessing...",
+    "skipped": "Skipped",
+    "copied": "✓ Copied",
+    "copyContent": "Copy Content",
+    "saving": "✓ Saving...",
+    "saveToFile": "Save to File",
+    "processedContent": "Processed Content",
+    "contentReady": "Content is ready to be saved",
+    "filesByTokenCount": "Files by Token Count",
+    "filePath": "File Path",
+    "noFileData": "No file data available",
+    "noProcessedContent": "No processed content yet",
+    "noProcessedContentHint": "Go to the Source tab to select files, then analyze and process them."
+  },
+  "fileTree": {
+    "collapseFolder": "Collapse folder",
+    "expandFolder": "Expand folder",
+    "collapseFolderWithName": "Collapse folder {{name}}",
+    "expandFolderWithName": "Expand folder {{name}}",
+    "selectAll": "Select All",
+    "selectedCount": "{{selected}} of {{total}} files selected",
+    "emptyTitle": "No files to display",
+    "emptyHint": "Select a directory to view files"
+  },
+  "errors": {
+    "selectRootAndFiles": "Please select a root directory and at least one file.",
+    "noValidFiles": "No valid files selected for analysis. Please select files within the current directory.",
+    "electronApiUnavailable": "Electron API is not available.",
+    "processingFailed": "An error occurred while processing the repository. Check the console for details.",
+    "noFilesSelectedForProcessing": "No files are selected for processing. Please go to the Source tab and select files.",
+    "refreshFailed": "An error occurred while refreshing content. Check the console for details.",
+    "noProcessedContentToSave": "No processed content to save.",
+    "saveFailed": "An error occurred while saving the file. Check the console for details."
+  }
+}
diff --git a/src/renderer/i18n/locales/es/common.json b/src/renderer/i18n/locales/es/common.json
new file mode 100644
index 0000000..3643f48
--- /dev/null
+++ b/src/renderer/i18n/locales/es/common.json
@@ -0,0 +1,127 @@
+{
+  "app": {
+    "name": "AI Code Fusion",
+    "github": "Ver en GitHub",
+    "dismissError": "Cerrar error"
+  },
+  "languageSelector": {
+    "label": "Idioma",
+    "en": "Inglés",
+    "es": "Español",
+    "fr": "Francés",
+    "de": "Alemán"
+  },
+  "darkMode": {
+    "switchToLight": "Cambiar a modo claro",
+    "switchToDark": "Cambiar a modo oscuro"
+  },
+  "tabs": {
+    "config": "Inicio",
+    "source": "Seleccionar archivos",
+    "processed": "Salida procesada"
+  },
+  "common": {
+    "files": "Archivos",
+    "tokens": "Tokens"
+  },
+  "config": {
+    "selectRootFolderPlaceholder": "Selecciona una carpeta raíz",
+    "browseDirectoryTitle": "Haz clic para buscar un directorio",
+    "selectFolder": "Seleccionar carpeta",
+    "fileFilteringTitle": "Filtrado de archivos",
+    "filterByExtensions": "Filtrar por extensiones",
+    "useExcludePatterns": "Usar patrones de exclusión",
+    "applyGitignoreRules": "Aplicar reglas de .gitignore",
+    "scanSecrets": "Escanear contenido en busca de secretos",
+    "excludeSuspiciousFiles": "Excluir archivos sospechosos",
+    "outputFormattingTitle": "Formato de salida",
+    "includeFileTree": "Incluir árbol de archivos en la salida",
+    "displayTokenCounts": "Mostrar conteo de tokens",
+    "exportFormat": "Formato de exportación",
+    "exportFormatMarkdown": "Markdown",
+    "exportFormatXml": "XML",
+    "providerSetupTitle": "Asistente de configuración de proveedor",
+    "providerSetupDescription": "Configura un proveedor de modelos y ejecuta una prueba de conexión antes de guardar.",
+    "provider": "Proveedor",
+    "selectProvider": "Seleccionar proveedor",
+    "model": "Modelo",
+    "modelPlaceholder": "p. ej. gpt-4o-mini",
+    "baseUrlOptional": "URL base (opcional)",
+    "apiKeyOptionalOllama": "API key (opcional para Ollama)",
+    "apiKeyPlaceholder": "Ingresa la API key del proveedor",
+    "testing": "Probando...",
+    "testConnection": "Probar conexión",
+    "autoSaveHint": "Los cambios se guardan automáticamente y se aplicarán al cambiar a la pestaña Fuente. Las estimaciones de tokens ayudan a optimizar el contexto para repositorios grandes.",
+    "saveConfig": "Guardar configuración",
+    "savedConfig": "✓ Guardado",
+    "includeExtensionsTitle": "Procesar solo archivos con estas extensiones",
+    "includeExtensionsHint": "Una extensión por línea (incluye el punto)",
+    "excludePatternsTitle": "Patrones de exclusión",
+    "excludePatternsHint": "Un patrón por línea (usando glob)",
+    "configSummary": "Configura qué tipos de archivo incluir y qué patrones excluir del análisis.",
+    "providerTestDisabled": "La prueba de conexión del proveedor está deshabilitada fuera del modo de desarrollo.",
+    "providerFixBeforeTesting": "Corrige la configuración del proveedor antes de probar la conexión.",
+    "providerUnavailable": "La prueba de conexión del proveedor no está disponible en esta compilación.",
+    "providerFixBeforeSaving": "Corrige la configuración del proveedor antes de guardar.",
+    "connectionTestFailed": "La prueba de conexión falló: {{message}}",
+    "validation": {
+      "selectProvider": "Selecciona un proveedor.",
+      "modelRequired": "El modelo es obligatorio.",
+      "apiKeyRequired": "La API key es obligatoria para este proveedor.",
+      "baseUrlProtocol": "La URL base debe usar http o https.",
+      "baseUrlValid": "La URL base debe ser válida."
+    }
+  },
+  "source": {
+    "processSelectedFiles": "Procesar archivos seleccionados",
+    "processingSelectedFiles": "Procesando...",
+    "selectingFiles": "Seleccionando archivos...",
+    "selectFilesAndFolders": "Seleccionar archivos y carpetas",
+    "loadingDirectory": "Cargando contenido del directorio...",
+    "selectRootFolderPlaceholder": "Selecciona una carpeta raíz",
+    "browseDirectoryTitle": "Haz clic para buscar un directorio",
+    "changeFolder": "Cambiar carpeta",
+    "refreshFileList": "Actualizar lista de archivos",
+    "refreshFileListTitle": "Actualizar la lista de archivos",
+    "clearSelection": "Limpiar selección",
+    "clearSelectionTitle": "Limpiar todos los archivos seleccionados",
+    "analyzingWait": "Analizando los archivos seleccionados, por favor espera..."
+  },
+  "processed": {
+    "refreshCode": "Actualizar código",
+    "refreshCodeTitle": "Recargar archivos seleccionados y regenerar la salida con el contenido más reciente",
+    "reprocessing": "Reprocesando...",
+    "skipped": "Omitidos",
+    "copied": "✓ Copiado",
+    "copyContent": "Copiar contenido",
+    "saving": "✓ Guardando...",
+    "saveToFile": "Guardar en archivo",
+    "processedContent": "Contenido procesado",
+    "contentReady": "El contenido está listo para guardarse",
+    "filesByTokenCount": "Archivos por conteo de tokens",
+    "filePath": "Ruta del archivo",
+    "noFileData": "No hay datos de archivos disponibles",
+    "noProcessedContent": "Todavía no hay contenido procesado",
+    "noProcessedContentHint": "Ve a la pestaña Fuente para seleccionar archivos y luego analizarlos y procesarlos."
+  },
+  "fileTree": {
+    "collapseFolder": "Contraer carpeta",
+    "expandFolder": "Expandir carpeta",
+    "collapseFolderWithName": "Contraer carpeta {{name}}",
+    "expandFolderWithName": "Expandir carpeta {{name}}",
+    "selectAll": "Seleccionar todo",
+    "selectedCount": "{{selected}} de {{total}} archivos seleccionados",
+    "emptyTitle": "No hay archivos para mostrar",
+    "emptyHint": "Selecciona un directorio para ver archivos"
+  },
+  "errors": {
+    "selectRootAndFiles": "Selecciona un directorio raíz y al menos un archivo.",
+    "noValidFiles": "No hay archivos válidos para analizar. Selecciona archivos dentro del directorio actual.",
+    "electronApiUnavailable": "La API de Electron no está disponible.",
+    "processingFailed": "Se produjo un error al procesar el repositorio. Revisa la consola para más detalles.",
+    "noFilesSelectedForProcessing": "No hay archivos seleccionados para procesar. Ve a la pestaña Fuente y selecciona archivos.",
+    "refreshFailed": "Se produjo un error al actualizar el contenido. Revisa la consola para más detalles.",
+    "noProcessedContentToSave": "No hay contenido procesado para guardar.",
+    "saveFailed": "Se produjo un error al guardar el archivo. Revisa la consola para más detalles."
+  }
+}
diff --git a/src/renderer/i18n/locales/fr/common.json b/src/renderer/i18n/locales/fr/common.json
new file mode 100644
index 0000000..e8c299c
--- /dev/null
+++ b/src/renderer/i18n/locales/fr/common.json
@@ -0,0 +1,127 @@
+{
+  "app": {
+    "name": "AI Code Fusion",
+    "github": "Voir sur GitHub",
+    "dismissError": "Fermer l'erreur"
+  },
+  "languageSelector": {
+    "label": "Langue",
+    "en": "Anglais",
+    "es": "Espagnol",
+    "fr": "Français",
+    "de": "Allemand"
+  },
+  "darkMode": {
+    "switchToLight": "Passer en mode clair",
+    "switchToDark": "Passer en mode sombre"
+  },
+  "tabs": {
+    "config": "Démarrer",
+    "source": "Sélectionner des fichiers",
+    "processed": "Sortie traitée"
+  },
+  "common": {
+    "files": "Fichiers",
+    "tokens": "Tokens"
+  },
+  "config": {
+    "selectRootFolderPlaceholder": "Sélectionnez un dossier racine",
+    "browseDirectoryTitle": "Cliquez pour parcourir un dossier",
+    "selectFolder": "Sélectionner un dossier",
+    "fileFilteringTitle": "Filtrage des fichiers",
+    "filterByExtensions": "Filtrer par extensions",
+    "useExcludePatterns": "Utiliser des motifs d'exclusion",
+    "applyGitignoreRules": "Appliquer les règles .gitignore",
+    "scanSecrets": "Scanner le contenu pour les secrets",
+    "excludeSuspiciousFiles": "Exclure les fichiers suspects",
+    "outputFormattingTitle": "Format de sortie",
+    "includeFileTree": "Inclure l'arborescence dans la sortie",
+    "displayTokenCounts": "Afficher le nombre de tokens",
+    "exportFormat": "Format d'export",
+    "exportFormatMarkdown": "Markdown",
+    "exportFormatXml": "XML",
+    "providerSetupTitle": "Assistant de configuration du fournisseur",
+    "providerSetupDescription": "Configurez un fournisseur de modèle et lancez un test de connexion avant d'enregistrer.",
+    "provider": "Fournisseur",
+    "selectProvider": "Sélectionner un fournisseur",
+    "model": "Modèle",
+    "modelPlaceholder": "ex. gpt-4o-mini",
+    "baseUrlOptional": "URL de base (optionnelle)",
+    "apiKeyOptionalOllama": "Clé API (optionnelle pour Ollama)",
+    "apiKeyPlaceholder": "Entrez la clé API du fournisseur",
+    "testing": "Test en cours...",
+    "testConnection": "Tester la connexion",
+    "autoSaveHint": "Les modifications sont enregistrées automatiquement et seront appliquées lors du passage à l'onglet Source. Les estimations de tokens aident à optimiser le contexte pour les grands dépôts.",
+    "saveConfig": "Enregistrer la configuration",
+    "savedConfig": "✓ Enregistré",
+    "includeExtensionsTitle": "Traiter uniquement les fichiers avec ces extensions",
+    "includeExtensionsHint": "Une extension par ligne (inclure le point)",
+    "excludePatternsTitle": "Motifs d'exclusion",
+    "excludePatternsHint": "Un motif par ligne (syntaxe glob)",
+    "configSummary": "Configurez les types de fichiers à inclure et les motifs à exclure de l'analyse.",
+    "providerTestDisabled": "Le test de connexion fournisseur est désactivé hors mode développement.",
+    "providerFixBeforeTesting": "Corrigez les paramètres du fournisseur avant de tester la connexion.",
+    "providerUnavailable": "Le test de connexion fournisseur n'est pas disponible dans cette version.",
+    "providerFixBeforeSaving": "Corrigez les paramètres du fournisseur avant d'enregistrer.",
+    "connectionTestFailed": "Échec du test de connexion : {{message}}",
+    "validation": {
+      "selectProvider": "Sélectionnez un fournisseur.",
+      "modelRequired": "Le modèle est requis.",
+      "apiKeyRequired": "La clé API est requise pour ce fournisseur.",
+      "baseUrlProtocol": "L'URL de base doit utiliser http ou https.",
+      "baseUrlValid": "L'URL de base doit être valide."
+    }
+  },
+  "source": {
+    "processSelectedFiles": "Traiter les fichiers sélectionnés",
+    "processingSelectedFiles": "Traitement...",
+    "selectingFiles": "Sélection des fichiers...",
+    "selectFilesAndFolders": "Sélectionner des fichiers et dossiers",
+    "loadingDirectory": "Chargement du contenu du dossier...",
+    "selectRootFolderPlaceholder": "Sélectionnez un dossier racine",
+    "browseDirectoryTitle": "Cliquez pour parcourir un dossier",
+    "changeFolder": "Changer de dossier",
+    "refreshFileList": "Actualiser la liste des fichiers",
+    "refreshFileListTitle": "Actualiser la liste des fichiers",
+    "clearSelection": "Effacer la sélection",
+    "clearSelectionTitle": "Effacer tous les fichiers sélectionnés",
+    "analyzingWait": "Analyse des fichiers sélectionnés, veuillez patienter..."
+  },
+  "processed": {
+    "refreshCode": "Actualiser le code",
+    "refreshCodeTitle": "Recharger les fichiers sélectionnés et régénérer la sortie avec le contenu le plus récent",
+    "reprocessing": "Nouveau traitement...",
+    "skipped": "Ignorés",
+    "copied": "✓ Copié",
+    "copyContent": "Copier le contenu",
+    "saving": "✓ Enregistrement...",
+    "saveToFile": "Enregistrer dans un fichier",
+    "processedContent": "Contenu traité",
+    "contentReady": "Le contenu est prêt à être enregistré",
+    "filesByTokenCount": "Fichiers par nombre de tokens",
+    "filePath": "Chemin du fichier",
+    "noFileData": "Aucune donnée de fichier disponible",
+    "noProcessedContent": "Aucun contenu traité pour le moment",
+    "noProcessedContentHint": "Allez dans l'onglet Source pour sélectionner des fichiers, puis les analyser et les traiter."
+  },
+  "fileTree": {
+    "collapseFolder": "Réduire le dossier",
+    "expandFolder": "Développer le dossier",
+    "collapseFolderWithName": "Réduire le dossier {{name}}",
+    "expandFolderWithName": "Développer le dossier {{name}}",
+    "selectAll": "Tout sélectionner",
+    "selectedCount": "{{selected}} sur {{total}} fichiers sélectionnés",
+    "emptyTitle": "Aucun fichier à afficher",
+    "emptyHint": "Sélectionnez un dossier pour afficher les fichiers"
+  },
+  "errors": {
+    "selectRootAndFiles": "Veuillez sélectionner un dossier racine et au moins un fichier.",
+    "noValidFiles": "Aucun fichier valide sélectionné pour l'analyse. Sélectionnez des fichiers dans le dossier actuel.",
+    "electronApiUnavailable": "L'API Electron n'est pas disponible.",
+    "processingFailed": "Une erreur s'est produite lors du traitement du dépôt. Consultez la console pour plus de détails.",
+    "noFilesSelectedForProcessing": "Aucun fichier sélectionné pour le traitement. Allez dans l'onglet Source et sélectionnez des fichiers.",
+    "refreshFailed": "Une erreur s'est produite lors de l'actualisation du contenu. Consultez la console pour plus de détails.",
+    "noProcessedContentToSave": "Aucun contenu traité à enregistrer.",
+    "saveFailed": "Une erreur s'est produite lors de l'enregistrement du fichier. Consultez la console pour plus de détails."
+  }
+}
diff --git a/src/renderer/i18n/settings.ts b/src/renderer/i18n/settings.ts
new file mode 100644
index 0000000..b6a5c55
--- /dev/null
+++ b/src/renderer/i18n/settings.ts
@@ -0,0 +1,59 @@
+export const SUPPORTED_LOCALES = ['en', 'es', 'fr', 'de'] as const;
+
+export type SupportedLocale = (typeof SUPPORTED_LOCALES)[number];
+
+export const DEFAULT_LOCALE: SupportedLocale = 'en';
+export const LOCALE_STORAGE_KEY = 'app.locale';
+
+export const isSupportedLocale = (value: string | null | undefined): value is SupportedLocale => {
+  return value !== null && value !== undefined && SUPPORTED_LOCALES.includes(value as SupportedLocale);
+};
+
+const normalizeLocale = (value: string): SupportedLocale | null => {
+  if (isSupportedLocale(value)) {
+    return value;
+  }
+
+  const baseLocale = value.split('-')[0];
+  if (isSupportedLocale(baseLocale)) {
+    return baseLocale;
+  }
+
+  return null;
+};
+
+export const getInitialLocale = (): SupportedLocale => {
+  if (typeof window === 'undefined') {
+    return DEFAULT_LOCALE;
+  }
+
+  const storedLocale = window.localStorage.getItem(LOCALE_STORAGE_KEY);
+  if (storedLocale) {
+    const normalizedStoredLocale = normalizeLocale(storedLocale);
+    if (normalizedStoredLocale) {
+      return normalizedStoredLocale;
+    }
+  }
+
+  const languageCandidates = [
+    ...(window.navigator.languages || []),
+    window.navigator.language,
+  ].filter(Boolean);
+
+  for (const candidate of languageCandidates) {
+    const normalizedLocale = normalizeLocale(candidate);
+    if (normalizedLocale) {
+      return normalizedLocale;
+    }
+  }
+
+  return DEFAULT_LOCALE;
+};
+
+export const persistLocale = (locale: SupportedLocale): void => {
+  if (typeof window === 'undefined') {
+    return;
+  }
+
+  window.localStorage.setItem(LOCALE_STORAGE_KEY, locale);
+};
diff --git a/src/renderer/index.tsx b/src/renderer/index.tsx
index ca1eb28..0622520 100644
--- a/src/renderer/index.tsx
+++ b/src/renderer/index.tsx
@@ -1,6 +1,7 @@
 import React from 'react';
 import { createRoot } from 'react-dom/client';
 
+import './i18n';
 import App from './components/App';
 
 const container = document.getElementById('app');
diff --git a/tests/catalog.md b/tests/catalog.md
index abbb64a..d97b39a 100644
--- a/tests/catalog.md
+++ b/tests/catalog.md
@@ -20,35 +20,37 @@ Purpose: quick map of what is covered, why it exists, and which command to run.
 
 ## Unit Tests
 
-| File                                                   | Primary Target                          | Key Use Cases                                                                                                                                          |
-| ------------------------------------------------------ | --------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------ |
-| `tests/unit/components/app.test.tsx`                   | `src/renderer/components/App.tsx`       | Tab switching, config load, directory selection, processing flow, error handling                                                                       |
-| `tests/unit/components/config-tab.test.tsx`            | `src/renderer/components/ConfigTab.tsx` | Config toggles/inputs, dev-only provider surface gating, provider validation/connection wiring, provider-config preservation, directory picker trigger |
-| `tests/unit/components/file-tree.test.tsx`             | `src/renderer/components/FileTree.tsx`  | Tree render, folder expand/collapse, select all, empty-state behavior                                                                                  |
-| `tests/unit/components/source-tab.test.tsx`            | `src/renderer/components/SourceTab.tsx` | Source tab input state, filter toggles, and event forwarding behavior                                                                                  |
-| `tests/unit/file-analyzer.test.ts`                     | `src/utils/file-analyzer.ts`            | Include/exclude rules, gitignore behavior, binary handling, error cases                                                                                |
-| `tests/unit/gitignore-parser.test.ts`                  | `src/utils/gitignore-parser.ts`         | Pattern parsing, negation behavior, caching, nested path handling                                                                                      |
-| `tests/unit/binary-detection.test.ts`                  | `src/utils/file-analyzer.ts`            | Binary signature detection, control-char thresholds, fallback-on-error behavior                                                                        |
-| `tests/unit/utils/filter-utils.test.ts`                | `src/utils/filter-utils.ts`             | Path normalization, extension filtering, custom excludes, gitignore precedence                                                                         |
-| `tests/unit/utils/secret-scanner.test.ts`              | `src/utils/secret-scanner.ts`           | Sensitive path detection, secret-pattern scanning, default-on safety toggles                                                                           |
-| `tests/unit/utils/fnmatch.test.ts`                     | `src/utils/fnmatch.ts`                  | Glob semantics: wildcards, classes, double-star, braces, path anchors                                                                                  |
-| `tests/unit/utils/export-format.test.ts`               | `src/utils/export-format.ts`            | Export format normalization, XML attribute escaping, CDATA-safe sanitization                                                                           |
-| `tests/unit/utils/content-processor.test.ts`           | `src/utils/content-processor.ts`        | Content assembly, binary skip logic, malformed input handling                                                                                          |
-| `tests/unit/utils/config-manager.test.ts`              | `src/utils/config-manager.ts`           | Default config load, parse failures, graceful fallback behavior                                                                                        |
-| `tests/unit/utils/token-counter.test.ts`               | `src/utils/token-counter.ts`            | Token counting basics, empty/null input handling                                                                                                       |
-| `tests/unit/scripts/security.test.js`                  | `scripts/lib/security.js`               | Command safety validation, Windows path acceptance for approved executables                                                                            |
-| `tests/unit/scripts/actions-freshness.test.js`         | `scripts/lib/actions-freshness.js`      | Workflow `uses:` reference parsing, pinning classification, freshness markdown report output                                                           |
-| `tests/unit/scripts/eslint-config.test.js`             | `eslint.config.js`                      | Guard scoped unicorn/sonarjs strict-pack configuration and test exclusions                                                                             |
-| `tests/unit/scripts/lint-gates.test.js`                | `package.json` + `eslint.config.js`     | Ensure lint/format gates include scripts + config coverage and staged-lint scope                                                                       |
-| `tests/unit/scripts/electron-eslint-rules.test.js`     | `eslint-rules/electron-security.js`     | Validate custom Electron safety lint rules (BrowserWindow flags, IPC channels, renderer bans)                                                          |
-| `tests/unit/scripts/sonar-options.test.js`             | `scripts/lib/sonar-options.js`          | Sonar scanner option merge behavior and CPD exclusion defaults                                                                                         |
-| `tests/unit/scripts/publish-stress-metrics.test.js`    | `scripts/publish-stress-metrics.js`     | Prometheus payload generation and Pushgateway publication safeguards                                                                                   |
-| `tests/unit/scripts/verify-prometheus-metrics.test.js` | `scripts/verify-prometheus-metrics.js`  | Prometheus scrape verification retries, timeouts, and parsing                                                                                          |
-| `tests/unit/scripts/perf-metrics-job.test.js`          | `scripts/run-perf-metrics-job.js`       | End-to-end performance job orchestration (stress, publish, verify)                                                                                     |
-| `tests/unit/scripts/validate-test-catalog.test.js`     | `scripts/validate-test-catalog.js`      | Catalog path validity and Jest discovery coverage checks                                                                                               |
-| `tests/unit/scripts/validate-changelog.test.js`        | `scripts/validate-changelog.js`         | Release heading/date format checks, allowed section headings, latest release section coverage                                                          |
-| `tests/unit/main/updater.test.ts`                      | `src/main/updater.ts`                   | Alpha/stable channel selection, platform gating, update-check result handling                                                                          |
-| `tests/unit/main/feature-flags.test.ts`                | `src/main/feature-flags.ts`             | OpenFeature normalization, env/remote merge rules, secure remote fetch behavior                                                                        |
+| File                                                   | Primary Target                                 | Key Use Cases                                                                                                                                          |
+| ------------------------------------------------------ | ---------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------ |
+| `tests/unit/components/app.test.tsx`                   | `src/renderer/components/App.tsx`              | Tab switching, config load, directory selection, processing flow, error handling                                                                       |
+| `tests/unit/components/config-tab.test.tsx`            | `src/renderer/components/ConfigTab.tsx`        | Config toggles/inputs, dev-only provider surface gating, provider validation/connection wiring, provider-config preservation, directory picker trigger |
+| `tests/unit/components/file-tree.test.tsx`             | `src/renderer/components/FileTree.tsx`         | Tree render, folder expand/collapse, select all, empty-state behavior                                                                                  |
+| `tests/unit/components/language-selector.test.tsx`     | `src/renderer/components/LanguageSelector.tsx` | Locale selector rendering, language switching, and localStorage persistence                                                                            |
+| `tests/unit/components/source-tab.test.tsx`            | `src/renderer/components/SourceTab.tsx`        | Source tab input state, filter toggles, and event forwarding behavior                                                                                  |
+| `tests/unit/i18n/locales-parity.test.ts`               | `src/renderer/i18n/locales/*/common.json`      | Locale key parity across EN/ES/FR/DE resources                                                                                                         |
+| `tests/unit/file-analyzer.test.ts`                     | `src/utils/file-analyzer.ts`                   | Include/exclude rules, gitignore behavior, binary handling, error cases                                                                                |
+| `tests/unit/gitignore-parser.test.ts`                  | `src/utils/gitignore-parser.ts`                | Pattern parsing, negation behavior, caching, nested path handling                                                                                      |
+| `tests/unit/binary-detection.test.ts`                  | `src/utils/file-analyzer.ts`                   | Binary signature detection, control-char thresholds, fallback-on-error behavior                                                                        |
+| `tests/unit/utils/filter-utils.test.ts`                | `src/utils/filter-utils.ts`                    | Path normalization, extension filtering, custom excludes, gitignore precedence                                                                         |
+| `tests/unit/utils/secret-scanner.test.ts`              | `src/utils/secret-scanner.ts`                  | Sensitive path detection, secret-pattern scanning, default-on safety toggles                                                                           |
+| `tests/unit/utils/fnmatch.test.ts`                     | `src/utils/fnmatch.ts`                         | Glob semantics: wildcards, classes, double-star, braces, path anchors                                                                                  |
+| `tests/unit/utils/export-format.test.ts`               | `src/utils/export-format.ts`                   | Export format normalization, XML attribute escaping, CDATA-safe sanitization                                                                           |
+| `tests/unit/utils/content-processor.test.ts`           | `src/utils/content-processor.ts`               | Content assembly, binary skip logic, malformed input handling                                                                                          |
+| `tests/unit/utils/config-manager.test.ts`              | `src/utils/config-manager.ts`                  | Default config load, parse failures, graceful fallback behavior                                                                                        |
+| `tests/unit/utils/token-counter.test.ts`               | `src/utils/token-counter.ts`                   | Token counting basics, empty/null input handling                                                                                                       |
+| `tests/unit/scripts/security.test.js`                  | `scripts/lib/security.js`                      | Command safety validation, Windows path acceptance for approved executables                                                                            |
+| `tests/unit/scripts/actions-freshness.test.js`         | `scripts/lib/actions-freshness.js`             | Workflow `uses:` reference parsing, pinning classification, freshness markdown report output                                                           |
+| `tests/unit/scripts/eslint-config.test.js`             | `eslint.config.js`                             | Guard scoped unicorn/sonarjs strict-pack configuration and test exclusions                                                                             |
+| `tests/unit/scripts/lint-gates.test.js`                | `package.json` + `eslint.config.js`            | Ensure lint/format gates include scripts + config coverage and staged-lint scope                                                                       |
+| `tests/unit/scripts/electron-eslint-rules.test.js`     | `eslint-rules/electron-security.js`            | Validate custom Electron safety lint rules (BrowserWindow flags, IPC channels, renderer bans)                                                          |
+| `tests/unit/scripts/sonar-options.test.js`             | `scripts/lib/sonar-options.js`                 | Sonar scanner option merge behavior and CPD exclusion defaults                                                                                         |
+| `tests/unit/scripts/publish-stress-metrics.test.js`    | `scripts/publish-stress-metrics.js`            | Prometheus payload generation and Pushgateway publication safeguards                                                                                   |
+| `tests/unit/scripts/verify-prometheus-metrics.test.js` | `scripts/verify-prometheus-metrics.js`         | Prometheus scrape verification retries, timeouts, and parsing                                                                                          |
+| `tests/unit/scripts/perf-metrics-job.test.js`          | `scripts/run-perf-metrics-job.js`              | End-to-end performance job orchestration (stress, publish, verify)                                                                                     |
+| `tests/unit/scripts/validate-test-catalog.test.js`     | `scripts/validate-test-catalog.js`             | Catalog path validity and Jest discovery coverage checks                                                                                               |
+| `tests/unit/scripts/validate-changelog.test.js`        | `scripts/validate-changelog.js`                | Release heading/date format checks, allowed section headings, latest release section coverage                                                          |
+| `tests/unit/main/updater.test.ts`                      | `src/main/updater.ts`                          | Alpha/stable channel selection, platform gating, update-check result handling                                                                          |
+| `tests/unit/main/feature-flags.test.ts`                | `src/main/feature-flags.ts`                    | OpenFeature normalization, env/remote merge rules, secure remote fetch behavior                                                                        |
 
 ## Integration Tests
 
@@ -76,9 +78,9 @@ Stress benchmark outputs:
 
 ## Electron E2E Tests
 
-| File                                      | Primary Target                                | Key Use Cases                                                                                                     |
-| ----------------------------------------- | --------------------------------------------- | ----------------------------------------------------------------------------------------------------------------- |
-| `tests/e2e/electron-process-flow.spec.ts` | Full renderer + preload + main-process wiring | Folder selection, file tree interaction, process flow, XML format handling, refresh-from-disk behavior, save flow |
+| File                                      | Primary Target                                | Key Use Cases                                                                                                                         |
+| ----------------------------------------- | --------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------- |
+| `tests/e2e/electron-process-flow.spec.ts` | Full renderer + preload + main-process wiring | Folder selection, file tree interaction, process flow, XML format handling, refresh-from-disk behavior, save flow, locale persistence |
 
 ## Visual Regression Signal
 
@@ -103,6 +105,8 @@ Stress benchmark outputs:
 - Renderer flow changes:
   - `tests/unit/components/app.test.tsx`
   - `tests/unit/components/config-tab.test.tsx`
+  - `tests/unit/components/language-selector.test.tsx`
+  - `tests/unit/i18n/locales-parity.test.ts`
   - `tests/e2e/electron-process-flow.spec.ts`
 - Main process / IPC changes:
   - `tests/integration/main-process/handlers.test.ts`
diff --git a/tests/e2e/electron-process-flow.spec.ts b/tests/e2e/electron-process-flow.spec.ts
index 70678f7..8b794c5 100644
--- a/tests/e2e/electron-process-flow.spec.ts
+++ b/tests/e2e/electron-process-flow.spec.ts
@@ -245,6 +245,11 @@ const test = base.extend<E2EFixtures>({
   page: async ({ electronApp }, use) => {
     const page = await electronApp.firstWindow();
     await page.waitForLoadState('domcontentloaded');
+    await page.evaluate(() => {
+      localStorage.setItem('app.locale', 'en');
+    });
+    await page.reload();
+    await page.waitForLoadState('domcontentloaded');
     await expect(page.getByRole('heading', { name: 'AI Code Fusion' })).toBeVisible();
     await use(page);
   },
@@ -323,3 +328,17 @@ test('saves processed output to disk through the native save flow', async ({ pag
   expect(savedContent).toContain('src/App.tsx');
   expect(savedContent).toContain('APP_MARKER_V1');
 });
+
+test('persists selected language across reloads', async ({ page }) => {
+  const languageSelector = page.getByTestId('language-selector');
+  await expect(languageSelector).toHaveValue('en');
+
+  await languageSelector.selectOption('es');
+  await expect(languageSelector).toHaveValue('es');
+  await expect(page.getByRole('tab', { name: 'Inicio' })).toBeVisible();
+
+  await page.reload();
+  await page.waitForLoadState('domcontentloaded');
+  await expect(page.getByTestId('language-selector')).toHaveValue('es');
+  await expect(page.getByRole('tab', { name: 'Inicio' })).toBeVisible();
+});
diff --git a/tests/setup.ts b/tests/setup.ts
index 5373288..63c375f 100644
--- a/tests/setup.ts
+++ b/tests/setup.ts
@@ -1,4 +1,11 @@
 import '@testing-library/jest-dom';
+import '../src/renderer/i18n';
+import i18n from '../src/renderer/i18n';
+
+beforeEach(async () => {
+  window.localStorage.setItem('app.locale', 'en');
+  await i18n.changeLanguage('en');
+});
 
 // Add a dummy test to avoid Jest warning about no tests
 describe('Setup validation', () => {
@@ -131,6 +138,7 @@ jest.mock('fs', () => ({
 // Mock console methods to reduce test noise
 global.console = {
   ...console,
+  info: jest.fn(),
   error: jest.fn(),
   warn: jest.fn(),
   log: jest.fn(),
diff --git a/tests/unit/components/language-selector.test.tsx b/tests/unit/components/language-selector.test.tsx
new file mode 100644
index 0000000..0b4fc0c
--- /dev/null
+++ b/tests/unit/components/language-selector.test.tsx
@@ -0,0 +1,39 @@
+import React from 'react';
+import { fireEvent, render, screen, waitFor } from '@testing-library/react';
+import '@testing-library/jest-dom';
+
+import LanguageSelector from '../../../src/renderer/components/LanguageSelector';
+import i18n from '../../../src/renderer/i18n';
+import { LOCALE_STORAGE_KEY } from '../../../src/renderer/i18n/settings';
+
+describe('LanguageSelector', () => {
+  beforeEach(async () => {
+    window.localStorage.clear();
+    await i18n.changeLanguage('en');
+  });
+
+  test('renders supported locale options', () => {
+    render(<LanguageSelector />);
+
+    const languageSelect = screen.getByTestId('language-selector');
+    expect(languageSelect).toHaveValue('en');
+
+    const optionValues = Array.from(screen.getAllByRole('option')).map((option) => option.getAttribute('value'));
+    expect(optionValues).toEqual(['en', 'es', 'fr', 'de']);
+  });
+
+  test('changes locale and persists it to localStorage', async () => {
+    const setItemSpy = jest.spyOn(Storage.prototype, 'setItem');
+    render(<LanguageSelector />);
+
+    fireEvent.change(screen.getByTestId('language-selector'), { target: { value: 'es' } });
+
+    await waitFor(() => {
+      expect(i18n.resolvedLanguage?.startsWith('es') || i18n.language.startsWith('es')).toBe(true);
+      expect(setItemSpy).toHaveBeenCalledWith(LOCALE_STORAGE_KEY, 'es');
+      expect(screen.getByLabelText('Idioma')).toBeInTheDocument();
+    });
+
+    setItemSpy.mockRestore();
+  });
+});
diff --git a/tests/unit/i18n/locales-parity.test.ts b/tests/unit/i18n/locales-parity.test.ts
new file mode 100644
index 0000000..e09fa85
--- /dev/null
+++ b/tests/unit/i18n/locales-parity.test.ts
@@ -0,0 +1,43 @@
+import deCommon from '../../../src/renderer/i18n/locales/de/common.json';
+import enCommon from '../../../src/renderer/i18n/locales/en/common.json';
+import esCommon from '../../../src/renderer/i18n/locales/es/common.json';
+import frCommon from '../../../src/renderer/i18n/locales/fr/common.json';
+
+type JsonRecord = Record<string, unknown>;
+
+const collectLeafKeys = (value: unknown, prefix = ''): string[] => {
+  if (value === null || typeof value !== 'object' || Array.isArray(value)) {
+    return prefix ? [prefix] : [];
+  }
+
+  const keys = Object.keys(value as JsonRecord);
+  if (keys.length === 0) {
+    return prefix ? [prefix] : [];
+  }
+
+  let result: string[] = [];
+  for (const key of keys) {
+    const nextPrefix = prefix ? `${prefix}.${key}` : key;
+    result = [...result, ...collectLeafKeys((value as JsonRecord)[key], nextPrefix)];
+  }
+
+  return result;
+};
+
+const getSortedUniqueLeafKeys = (locale: JsonRecord): string[] => {
+  return [...new Set(collectLeafKeys(locale))].sort();
+};
+
+describe('i18n locale key parity', () => {
+  const englishKeys = getSortedUniqueLeafKeys(enCommon as JsonRecord);
+
+  test.each([
+    ['es', esCommon],
+    ['fr', frCommon],
+    ['de', deCommon],
+  ])('%s locale has exactly the same keys as en', (localeCode, locale) => {
+    const localeKeys = getSortedUniqueLeafKeys(locale as JsonRecord);
+
+    expect(localeKeys).toEqual(englishKeys);
+  });
+});

From 21c5ac7fee065e03a13d61a84fda32da1661cdcd Mon Sep 17 00:00:00 2001
From: Mehdi <mehdi@blagui.com>
Date: Sat, 14 Feb 2026 18:21:31 +0000
Subject: [PATCH 2/3] docs(agents): enforce local-only docs/plan workflow

---
 AGENTS.md | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/AGENTS.md b/AGENTS.md
index 11e93e7..1f8c009 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -6,20 +6,27 @@ Lightweight rules for automated agents and contributors in this repository.
 
 - Keep changes focused and minimal.
 - Do not make unrelated refactors.
+- Keep project code structure with clear scoping for each scoping, don't mixup up code.
+- planning docs are in docs/plan and never committed, they can live in backlog or local files.
 
 ## Tests and Quality
 
 - Before proposing merge-ready changes, run:
   - `npm run lint`
   - `npm test -- --runInBand`
+  - ensure all linting warning errors are cleared
 - If UI behavior/layout is changed, also run:
   - `npm run qa:screenshot`
 - Do not mark work complete while required CI checks are failing.
+- Read PR comments, evaluate and fact check,
 - Use `tests/catalog.md` as the source of truth for test targets and use cases.
+- Locally before pushing run make sonar and ensure to fix all issues.
+- After pushing a PR, check SonarCloud report again and clear any reported issues.
 
 ## `.env` Policy (Local-Only)
 
-- `.env` in this repo is for local development bootstrap only.
+- `.env` in this repo is for local development bootstrap only, and we commit it as it's not sensitive.
+- ignore agents review for vault-agent-env.hcl over hardcoded env.
 - Keep `.env` tracked; do not remove/rename it.
 - Do not over-engineer or heavily refactor `.env` for non-local use.
 - Never commit secrets/tokens in `.env`.
@@ -30,3 +37,6 @@ Lightweight rules for automated agents and contributors in this repository.
 - Treat `.env` changes as sensitive even when local-only.
 - Ensure `.env` updates are minimal, intentional, and documented in PR notes.
 - Reject any hardcoded secret, token, or credential exposure in code, docs, or PR text.
+- Check agents/review over current use, if the tool is used in dev only, the evaluation the benefit risk for the change.
+- Focus in feedback on our core app features.
+- Use feature flag, if the new feature require other steps to complete and set it enabled only in dev mode so we can test it.

From 153168a81d4f7a570fa8aa8b3c2e1dcc54c0c95a Mon Sep 17 00:00:00 2001
From: Mehdi <mehdi@blagui.com>
Date: Sat, 14 Feb 2026 18:31:39 +0000
Subject: [PATCH 3/3] fix(i18n): address PR feedback for locale handling and
 error reactivity

---
 AGENTS.md                                    | 22 +++++-----
 src/renderer/components/App.tsx              |  5 ++-
 src/renderer/components/LanguageSelector.tsx | 14 ++++---
 src/renderer/context/AppContext.tsx          | 43 +++++++++++++++-----
 4 files changed, 57 insertions(+), 27 deletions(-)

diff --git a/AGENTS.md b/AGENTS.md
index 1f8c009..54b00fd 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -6,27 +6,27 @@ Lightweight rules for automated agents and contributors in this repository.
 
 - Keep changes focused and minimal.
 - Do not make unrelated refactors.
-- Keep project code structure with clear scoping for each scoping, don't mixup up code.
-- planning docs are in docs/plan and never committed, they can live in backlog or local files.
+- Keep project code structure with clear scoping for each module; do not mix up code.
+- Planning docs are in `docs/plan` and are never committed. They can live in backlog or local files.
 
 ## Tests and Quality
 
 - Before proposing merge-ready changes, run:
   - `npm run lint`
   - `npm test -- --runInBand`
-  - ensure all linting warning errors are cleared
+  - Ensure all linting warnings and errors are cleared.
 - If UI behavior/layout is changed, also run:
   - `npm run qa:screenshot`
 - Do not mark work complete while required CI checks are failing.
-- Read PR comments, evaluate and fact check,
+- Read PR comments, evaluate them, and fact-check.
 - Use `tests/catalog.md` as the source of truth for test targets and use cases.
-- Locally before pushing run make sonar and ensure to fix all issues.
-- After pushing a PR, check SonarCloud report again and clear any reported issues.
+- Locally before pushing, run `make sonar` and fix all issues.
+- After pushing a PR, check the SonarCloud report again and clear any reported issues.
 
 ## `.env` Policy (Local-Only)
 
-- `.env` in this repo is for local development bootstrap only, and we commit it as it's not sensitive.
-- ignore agents review for vault-agent-env.hcl over hardcoded env.
+- `.env` in this repo is for local development bootstrap only, and we commit it because it is not sensitive.
+- Ignore agent review warnings for `vault-agent-env.hcl` hardcoded env.
 - Keep `.env` tracked; do not remove/rename it.
 - Do not over-engineer or heavily refactor `.env` for non-local use.
 - Never commit secrets/tokens in `.env`.
@@ -37,6 +37,6 @@ Lightweight rules for automated agents and contributors in this repository.
 - Treat `.env` changes as sensitive even when local-only.
 - Ensure `.env` updates are minimal, intentional, and documented in PR notes.
 - Reject any hardcoded secret, token, or credential exposure in code, docs, or PR text.
-- Check agents/review over current use, if the tool is used in dev only, the evaluation the benefit risk for the change.
-- Focus in feedback on our core app features.
-- Use feature flag, if the new feature require other steps to complete and set it enabled only in dev mode so we can test it.
+- Check agent reviews in context; if a tool is used only in dev, evaluate benefit and risk for the change.
+- Focus feedback on our core app features.
+- Use feature flags if a new feature requires additional steps, and enable it only in dev mode for testing.
diff --git a/src/renderer/components/App.tsx b/src/renderer/components/App.tsx
index 9baf3f1..e9d84d8 100755
--- a/src/renderer/components/App.tsx
+++ b/src/renderer/components/App.tsx
@@ -17,10 +17,13 @@ const ErrorBanner = () => {
   const { t } = useTranslation();
 
   if (!appError) return null;
+  const translatedMessage = appError.translationKey
+    ? t(appError.translationKey, appError.translationOptions)
+    : appError.message;
 
   return (
     <div className='flex items-center justify-between bg-red-50 dark:bg-red-900/30 border-b border-red-200 dark:border-red-800 px-4 py-2 text-sm text-red-800 dark:text-red-200'>
-      <span>{appError.message}</span>
+      <span>{translatedMessage}</span>
       <button
         onClick={dismissError}
         className='ml-4 shrink-0 text-red-600 dark:text-red-300 hover:text-red-800 dark:hover:text-red-100'
diff --git a/src/renderer/components/LanguageSelector.tsx b/src/renderer/components/LanguageSelector.tsx
index 236f9c9..83f77eb 100644
--- a/src/renderer/components/LanguageSelector.tsx
+++ b/src/renderer/components/LanguageSelector.tsx
@@ -1,15 +1,19 @@
 import React from 'react';
 import { useTranslation } from 'react-i18next';
 
-import { persistLocale, SUPPORTED_LOCALES, type SupportedLocale } from '../i18n/settings';
+import {
+  DEFAULT_LOCALE,
+  isSupportedLocale,
+  persistLocale,
+  SUPPORTED_LOCALES,
+  type SupportedLocale,
+} from '../i18n/settings';
 
 const LanguageSelector = () => {
   const { i18n, t } = useTranslation();
 
-  const resolvedLanguage = (i18n.resolvedLanguage || i18n.language || 'en').split('-')[0];
-  const selectedLocale = (SUPPORTED_LOCALES.includes(resolvedLanguage as SupportedLocale)
-    ? resolvedLanguage
-    : 'en') as SupportedLocale;
+  const baseLanguage = (i18n.resolvedLanguage || i18n.language || DEFAULT_LOCALE).split('-')[0];
+  const selectedLocale = isSupportedLocale(baseLanguage) ? baseLanguage : DEFAULT_LOCALE;
 
   const handleLanguageChange = async (event: React.ChangeEvent<HTMLSelectElement>) => {
     const nextLocale = event.target.value as SupportedLocale;
diff --git a/src/renderer/context/AppContext.tsx b/src/renderer/context/AppContext.tsx
index b07e13c..24ca1b4 100644
--- a/src/renderer/context/AppContext.tsx
+++ b/src/renderer/context/AppContext.tsx
@@ -46,6 +46,8 @@ type ProcessingOptions = {
 
 type AppError = {
   message: string;
+  translationKey?: string;
+  translationOptions?: Record<string, string | number>;
   timestamp: number;
 };
 
@@ -167,9 +169,30 @@ export const AppProvider = ({ children }: AppProviderProps) => {
   const appWindow = globalThis as Window & typeof globalThis;
   const electronAPI = appWindow.electronAPI;
 
-  const showError = useCallback((message: string) => {
-    setAppError({ message, timestamp: Date.now() });
-  }, []);
+  const showError = useCallback(
+    (
+      error:
+        | string
+        | {
+            translationKey: string;
+            translationOptions?: Record<string, string | number>;
+            message?: string;
+          }
+    ) => {
+      if (typeof error === 'string') {
+        setAppError({ message: error, timestamp: Date.now() });
+        return;
+      }
+
+      setAppError({
+        message: error.message ?? '',
+        translationKey: error.translationKey,
+        translationOptions: error.translationOptions,
+        timestamp: Date.now(),
+      });
+    },
+    []
+  );
 
   const dismissError = useCallback(() => {
     setAppError(null);
@@ -438,7 +461,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
   const handleAnalyze = useCallback(async (): Promise<AnalyzeRepositoryResult | undefined> => {
     const selectedFilesArray = [...selectedFiles];
     if (!rootPath || selectedFilesArray.length === 0) {
-      showError(i18n.t('errors.selectRootAndFiles'));
+      showError({ translationKey: 'errors.selectRootAndFiles' });
       return undefined;
     }
 
@@ -453,7 +476,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
       });
 
       if (validFiles.length === 0) {
-        showError(i18n.t('errors.noValidFiles'));
+        showError({ translationKey: 'errors.noValidFiles' });
         return undefined;
       }
 
@@ -503,7 +526,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     } catch (error) {
       const processedError = ensureError(error);
       console.error('Error processing repository:', processedError);
-      showError(i18n.t('errors.processingFailed'));
+      showError({ translationKey: 'errors.processingFailed' });
       throw processedError;
     }
   }, [selectedFiles, rootPath, configContent, appWindow, showError]);
@@ -512,7 +535,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     try {
       const selectedFilesArray = [...selectedFiles];
       if (!rootPath || selectedFilesArray.length === 0) {
-        showError(i18n.t('errors.noFilesSelectedForProcessing'));
+        showError({ translationKey: 'errors.noFilesSelectedForProcessing' });
         return null;
       }
 
@@ -558,14 +581,14 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     } catch (error) {
       const processedError = ensureError(error);
       console.error('Error refreshing processed content:', processedError);
-      showError(i18n.t('errors.refreshFailed'));
+      showError({ translationKey: 'errors.refreshFailed' });
       throw processedError;
     }
   }, [selectedFiles, rootPath, configContent, appWindow, processingOptions, showError]);
 
   const handleSaveOutput = useCallback(async () => {
     if (!processedResult) {
-      showError(i18n.t('errors.noProcessedContentToSave'));
+      showError({ translationKey: 'errors.noProcessedContentToSave' });
       return;
     }
 
@@ -578,7 +601,7 @@ export const AppProvider = ({ children }: AppProviderProps) => {
     } catch (error) {
       const processedError = ensureError(error);
       console.error('Error saving file:', processedError);
-      showError(i18n.t('errors.saveFailed'));
+      showError({ translationKey: 'errors.saveFailed' });
     }
   }, [processedResult, appWindow, rootPath, showError]);