diff --git a/AGENTS.md b/AGENTS.md
index 11e93e7..54b00fd 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 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 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 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 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.
+- `.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`.
@@ -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 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/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..e9d84d8 100755
--- a/src/renderer/components/App.tsx
+++ b/src/renderer/components/App.tsx
@@ -1,26 +1,33 @@
 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;
+  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'
-        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 +38,7 @@ const ErrorBanner = () => {
 };
 
 const AppContent = () => {
+  const { t } = useTranslation();
   const {
     activeTab,
     rootPath,
@@ -59,6 +67,7 @@ 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 +76,12 @@ 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 +110,7 @@ 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..83f77eb
--- /dev/null
+++ b/src/renderer/components/LanguageSelector.tsx
@@ -0,0 +1,46 @@
+import React from 'react';
+import { useTranslation } from 'react-i18next';
+
+import {
+  DEFAULT_LOCALE,
+  isSupportedLocale,
+  persistLocale,
+  SUPPORTED_LOCALES,
+  type SupportedLocale,
+} from '../i18n/settings';
+
+const LanguageSelector = () => {
+  const { i18n, t } = useTranslation();
+
+  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;
+    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..24ca1b4 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,
@@ -45,6 +46,8 @@ type ProcessingOptions = {
 
 type AppError = {
   message: string;
+  translationKey?: string;
+  translationOptions?: Record<string, string | number>;
   timestamp: number;
 };
 
@@ -166,9 +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);
@@ -437,7 +461,7 @@ 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({ translationKey: 'errors.selectRootAndFiles' });
       return undefined;
     }
 
@@ -452,14 +476,12 @@ export const AppProvider = ({ children }: AppProviderProps) => {
       });
 
       if (validFiles.length === 0) {
-        showError(
-          'No valid files selected for analysis. Please select files within the current directory.'
-        );
+        showError({ translationKey: '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 +526,7 @@ 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({ translationKey: 'errors.processingFailed' });
       throw processedError;
     }
   }, [selectedFiles, rootPath, configContent, appWindow, showError]);
@@ -513,14 +535,12 @@ 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({ translationKey: '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 +581,14 @@ 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({ translationKey: 'errors.refreshFailed' });
       throw processedError;
     }
   }, [selectedFiles, rootPath, configContent, appWindow, processingOptions, showError]);
 
   const handleSaveOutput = useCallback(async () => {
     if (!processedResult) {
-      showError('No processed content to save.');
+      showError({ translationKey: 'errors.noProcessedContentToSave' });
       return;
     }
 
@@ -581,7 +601,7 @@ 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({ translationKey: '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);
+  });
+});