diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
new file mode 100644
index 000000000000..91169c45e453
--- /dev/null
+++ b/.github/copilot-instructions.md
@@ -0,0 +1,42 @@
+# Copilot Instructions for Decap CMS Monorepo
+
+Use this guide to be productive quickly. Keep changes minimal and scoped to the relevant package.
+
+## Overview
+- Monorepo under `packages/*` (core, app, backends, widgets, UI), orchestrated by Nx + Lerna; bundles via webpack in each package.
+- Core domain lives in `packages/decap-cms-core` (collections, entries, formats, i18n, search, backend abstraction).
+- The `Backend` class (`packages/decap-cms-core/src/backend.ts`) orchestrates entry CRUD, i18n grouping, media, search, and editorial workflow across provider backends.
+- App bundle is `packages/decap-cms` (UMD browser build) used by the demo under `dev-test/`.
+
+## Build, Run, Test
+- Dev watch all packages: `npm run develop` (Nx run-many; excludes `decap-server`).
+- Build all: `npm run build` (ESM then package builds); preview: `npm run build-preview`.
+- Unit tests: `npm run test:unit` (Jest, jsdom). Full CI set: `npm run test:ci`.
+- E2E (Cypress): headless `npm run test:e2e`; interactive `npm run test:e2e:dev`.
+- Demo site for manual testing: `npm run test:e2e:serve` then open `http://localhost:8080` (serves `dev-test/`).
+- Lint/format: `npm run lint`, `npm run format`. Type-check: `npm run type-check`.
+
+## Core Hotspots (start here)
+- Entry lifecycle: `decap-cms-core/src/backend.ts` – `processEntries`, `listEntries`, `listAllEntries`, `entryWithFormat`, `entryToRaw`, i18n helpers usage.
+- Collection rules: `decap-cms-core/src/reducers/collections.*` – selectors like `selectEntryPath`, `selectFolderEntryExtension`, nested/index behavior.
+- Formats: `decap-cms-core/src/formats/` – frontmatter/Markdown/YAML parsing/serialization.
+- i18n: `decap-cms-core/src/lib/i18n.*` – `hasI18n`, `groupEntries`, `getI18nFilesDepth`, `I18N_STRUCTURE`.
+- Registry: `decap-cms-core/src/lib/registry.*` – registers backends, widgets, media libraries.
+- Build config: `scripts/webpack.js` – outputs (UMD/CJS), externals from `peerDependencies`, source maps.
+
+## Patterns & Conventions
+- Immutable data: reducers/selectors use Immutable.js `Map/List`. Use selectors from `core/src/reducers/**` instead of direct shape access.
+- Format I/O: always use `resolveFormat`, `entryWithFormat`, and `entryToRaw` for parsing/serialization. Don’t hand-roll frontmatter.
+- i18n awareness: when listing/persisting, account for i18n files/folders via helpers (see `lib/i18n`).
+- Backends contract: implementations in `packages/decap-cms-backend-*` must support methods used by `Backend` (e.g., `entriesByFolder|Files`, `getEntry`, `persistEntry`, `getMedia`, optional `allEntriesByFolder`, `getDeployPreview`, editorial workflow methods). Confirm call sites in `core/src/backend.ts`.
+- Bundling: keep runtime deps vs `peerDependencies` correct; peers are externalized in UMD builds by `scripts/webpack.js`.
+
+## Integration Points
+- Local git proxy for dev: `packages/decap-server` (`npx decap-server` in a site repo, then set `backend: name: proxy`).
+- Cypress fixtures recording: see `cypress/Readme.md` and `mock:server:*` scripts; requires `.env` with provider tokens.
+
+## Practical Tips for Agents
+- Scope changes to a single package unless a cross-package change is intentional.
+- Preserve function signatures and Immutable types; convert to plain objects only where expected.
+- If changing serialization or path/slug logic, update both read and write paths and run E2E.
+- Feature-gate new backend capabilities and extend the interface used by `Backend` before implementing providers.
diff --git a/packages/decap-cms-core/src/backend.ts b/packages/decap-cms-core/src/backend.ts
index c998d645041d..9572b40c648e 100644
--- a/packages/decap-cms-core/src/backend.ts
+++ b/packages/decap-cms-core/src/backend.ts
@@ -521,9 +521,32 @@ export class Backend {
         },
       ),
     );
-    const formattedEntries = entries.map(this.entryWithFormat(collection));
-    // If this collection has a "filter" property, filter entries accordingly
+
+    let formattedEntries = entries.map(this.entryWithFormat(collection));
     const collectionFilter = collection.get('filter');
+    const i18nInfo = getI18nInfo(collection) as I18nInfo;
+    const isSingleFileI18n = i18nInfo && i18nInfo.structure === I18N_STRUCTURE.SINGLE_FILE;
+
+    if (isSingleFileI18n && collectionFilter) {
+      // For each entry, dig into the default locale and copy the filter key to root level for filtering
+      const currentLocale = i18nInfo.defaultLocale;
+      const filterField = collectionFilter.get('field');
+
+      formattedEntries = formattedEntries.map(entry => {
+        if (!entry.data || typeof entry.data !== 'object') return entry;
+        const localeData = entry.data[currentLocale];
+        if (!localeData || !(filterField in localeData)) return entry;
+        // Copy the filter field from localeData to root level
+        return {
+          ...entry,
+          data: {
+            ...entry.data,
+            [filterField]: localeData[filterField],
+          },
+        };
+      });
+    }
+
     const filteredEntries = collectionFilter
       ? this.filterEntries({ entries: formattedEntries }, collectionFilter)
       : formattedEntries;