diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5d70a16e..dd88e3d5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,7 @@ and this project adheres to
 ## Fixed
 
 - 🐛(backend) require right to manage document accesses to see invitations #369
+- 🐛(i18n) same frontend and backend language using shared cookies #365
 - 🐛(frontend) add default toolbar buttons #355
 
 
@@ -27,7 +28,7 @@ and this project adheres to
 
 - ✨AI to doc editor #250
 - ✨(backend) allow uploading more types of attachments #309
-- ✨(frontend) add buttons to copy document to clipboard as HTML/Markdown #300
+- ✨(frontend) add buttons to copy document to clipboard as HTML/Markdown #318
 
 ## Changed
 
diff --git a/src/backend/impress/settings.py b/src/backend/impress/settings.py
index 8a6fa8e0..394bbc83 100755
--- a/src/backend/impress/settings.py
+++ b/src/backend/impress/settings.py
@@ -223,6 +223,7 @@ class Base(Configuration):
 
     # Languages
     LANGUAGE_CODE = values.Value("en-us")
+    LANGUAGE_COOKIE_NAME = "docs_language"  # cookie & language is set from frontend
 
     DRF_NESTED_MULTIPART_PARSER = {
         # output of parser is converted to querydict
diff --git a/src/frontend/apps/e2e/__tests__/app-impress/language.spec.ts b/src/frontend/apps/e2e/__tests__/app-impress/language.spec.ts
index cc787605..3ed6317a 100644
--- a/src/frontend/apps/e2e/__tests__/app-impress/language.spec.ts
+++ b/src/frontend/apps/e2e/__tests__/app-impress/language.spec.ts
@@ -25,4 +25,38 @@ test.describe('Language', () => {
       }),
     ).toBeVisible();
   });
+
+  test('checks that backend uses the same language as the frontend', async ({
+    page,
+  }) => {
+    // Helper function to intercept and assert 404 response
+    const check404Response = async (expectedDetail: string) => {
+      const expectedBackendResponse = page.waitForResponse(
+        (response) =>
+          response.url().includes('/api') &&
+          response.url().includes('non-existent-doc-uuid') &&
+          response.status() === 404,
+      );
+
+      // Trigger the specific 404 XHR response by navigating to a non-existent document
+      await page.goto('/docs/non-existent-doc-uuid');
+
+      // Assert that the intercepted error message is in the expected language
+      const interceptedBackendResponse = await expectedBackendResponse;
+      expect(await interceptedBackendResponse.json()).toStrictEqual({
+        detail: expectedDetail,
+      });
+    };
+
+    // Check for English 404 response
+    await check404Response('Not found.');
+
+    // Switch language to French
+    const header = page.locator('header').first();
+    await header.getByRole('combobox').getByText('English').click();
+    await header.getByRole('option', { name: 'Français' }).click();
+
+    // Check for French 404 response
+    await check404Response('Pas trouvé.');
+  });
 });
diff --git a/src/frontend/apps/impress/package.json b/src/frontend/apps/impress/package.json
index 8f3a17bd..66740a91 100644
--- a/src/frontend/apps/impress/package.json
+++ b/src/frontend/apps/impress/package.json
@@ -23,6 +23,7 @@
     "@openfun/cunningham-react": "2.9.4",
     "@tanstack/react-query": "5.59.15",
     "i18next": "23.16.2",
+    "i18next-browser-languagedetector": "8.0.0",
     "idb": "8.0.0",
     "lodash": "4.17.21",
     "luxon": "3.5.0",
diff --git a/src/frontend/apps/impress/src/i18n/__tests__/utils.spec.ts b/src/frontend/apps/impress/src/i18n/__tests__/utils.spec.ts
deleted file mode 100644
index 38cee3d6..00000000
--- a/src/frontend/apps/impress/src/i18n/__tests__/utils.spec.ts
+++ /dev/null
@@ -1,55 +0,0 @@
-import { LANGUAGE_LOCAL_STORAGE } from '../conf';
-import { getLanguage, splitLocaleCode } from '../utils';
-
-describe('i18n utils', () => {
-  afterEach(() => {
-    localStorage.removeItem(LANGUAGE_LOCAL_STORAGE);
-  });
-
-  it('checks language code is correctly splitted', () => {
-    expect(splitLocaleCode('fr_FR')).toEqual({ language: 'fr', region: 'FR' });
-    expect(splitLocaleCode('en_US')).toEqual({ language: 'en', region: 'US' });
-    expect(splitLocaleCode('en')).toEqual({
-      language: 'en',
-      region: undefined,
-    });
-    expect(splitLocaleCode('fr-FR')).toEqual({ language: 'fr', region: 'FR' });
-    expect(splitLocaleCode('en-US')).toEqual({ language: 'en', region: 'US' });
-  });
-
-  it('checks that we get expected language from local storage', () => {
-    localStorage.setItem(LANGUAGE_LOCAL_STORAGE, 'fr_FR');
-    expect(getLanguage()).toEqual('fr');
-    localStorage.removeItem(LANGUAGE_LOCAL_STORAGE);
-
-    localStorage.setItem(LANGUAGE_LOCAL_STORAGE, 'en_FR');
-    expect(getLanguage()).toEqual('en');
-
-    localStorage.setItem(LANGUAGE_LOCAL_STORAGE, 'xx_XX');
-    expect(getLanguage()).toEqual('fr');
-  });
-
-  it('checks that we get expected language from browser', () => {
-    Object.defineProperty(navigator, 'language', {
-      value: 'fr',
-      writable: false,
-      configurable: true,
-    });
-
-    expect(getLanguage()).toEqual('fr');
-
-    Object.defineProperty(navigator, 'language', {
-      value: 'en',
-      writable: false,
-      configurable: true,
-    });
-    expect(getLanguage()).toEqual('en');
-
-    Object.defineProperty(navigator, 'language', {
-      value: 'xx',
-      writable: false,
-      configurable: true,
-    });
-    expect(getLanguage()).toEqual('fr');
-  });
-});
diff --git a/src/frontend/apps/impress/src/i18n/conf.ts b/src/frontend/apps/impress/src/i18n/conf.ts
index d58f5730..2ecc8a22 100644
--- a/src/frontend/apps/impress/src/i18n/conf.ts
+++ b/src/frontend/apps/impress/src/i18n/conf.ts
@@ -2,5 +2,5 @@ export const LANGUAGES_ALLOWED: { [key: string]: string } = {
   en: 'English',
   fr: 'Français',
 };
-export const LANGUAGE_LOCAL_STORAGE = 'impress-language';
-export const BASE_LANGUAGE = 'fr';
+export const LANGUAGE_COOKIE_NAME = 'docs_language';
+export const BASE_LANGUAGE = 'en';
diff --git a/src/frontend/apps/impress/src/i18n/initI18n.ts b/src/frontend/apps/impress/src/i18n/initI18n.ts
index f5a8f4be..e61b2dbc 100644
--- a/src/frontend/apps/impress/src/i18n/initI18n.ts
+++ b/src/frontend/apps/impress/src/i18n/initI18n.ts
@@ -1,15 +1,23 @@
 import i18n from 'i18next';
+import LanguageDetector from 'i18next-browser-languagedetector';
 import { initReactI18next } from 'react-i18next';
 
-import { LANGUAGES_ALLOWED, LANGUAGE_LOCAL_STORAGE } from './conf';
+import { BASE_LANGUAGE, LANGUAGES_ALLOWED, LANGUAGE_COOKIE_NAME } from './conf';
 import resources from './translations.json';
-import { getLanguage } from './utils';
 
 i18n
+  .use(LanguageDetector)
   .use(initReactI18next)
   .init({
     resources,
-    lng: getLanguage(),
+    fallbackLng: BASE_LANGUAGE,
+    supportedLngs: Object.keys(LANGUAGES_ALLOWED),
+    detection: {
+      order: ['cookie', 'navigator'], // detection order
+      caches: ['cookie'], // Use cookies to store the language preference
+      lookupCookie: LANGUAGE_COOKIE_NAME,
+      cookieMinutes: 525600, // Expires after one year
+    },
     interpolation: {
       escapeValue: false,
     },
@@ -20,11 +28,4 @@ i18n
     throw new Error('i18n initialization failed');
   });
 
-// Save language in local storage
-i18n.on('languageChanged', (lng) => {
-  if (typeof window !== 'undefined') {
-    localStorage.setItem(LANGUAGE_LOCAL_STORAGE, lng);
-  }
-});
-
 export default i18n;
diff --git a/src/frontend/apps/impress/src/i18n/utils.ts b/src/frontend/apps/impress/src/i18n/utils.ts
deleted file mode 100644
index edf51715..00000000
--- a/src/frontend/apps/impress/src/i18n/utils.ts
+++ /dev/null
@@ -1,28 +0,0 @@
-import {
-  BASE_LANGUAGE,
-  LANGUAGES_ALLOWED,
-  LANGUAGE_LOCAL_STORAGE,
-} from './conf';
-
-export const splitLocaleCode = (language: string) => {
-  const locale = language.split(/[-_]/);
-  return {
-    language: locale[0],
-    region: locale.length === 2 ? locale[1] : undefined,
-  };
-};
-
-export const getLanguage = () => {
-  if (typeof window === 'undefined') {
-    return BASE_LANGUAGE;
-  }
-
-  const languageStore =
-    localStorage.getItem(LANGUAGE_LOCAL_STORAGE) || navigator?.language;
-
-  const language = splitLocaleCode(languageStore).language;
-
-  return Object.keys(LANGUAGES_ALLOWED).includes(language)
-    ? language
-    : BASE_LANGUAGE;
-};
diff --git a/src/frontend/yarn.lock b/src/frontend/yarn.lock
index 427d00da..4909a50c 100644
--- a/src/frontend/yarn.lock
+++ b/src/frontend/yarn.lock
@@ -7167,6 +7167,13 @@ human-signals@^2.1.0:
   resolved "https://registry.yarnpkg.com/human-signals/-/human-signals-2.1.0.tgz#dc91fcba42e4d06e4abaed33b3e7a3c02f514ea0"
   integrity sha512-B4FFZ6q/T2jhhksgkbEW3HBvWIfDW85snkQgawt07S7J5QXTk6BkNV+0yAeZrM5QpMAdYlocGoljn0sJ/WQkFw==
 
+i18next-browser-languagedetector@8.0.0:
+  version "8.0.0"
+  resolved "https://registry.yarnpkg.com/i18next-browser-languagedetector/-/i18next-browser-languagedetector-8.0.0.tgz#b6fdd9b43af67c47f2c26c9ba27710a1eaf31e2f"
+  integrity sha512-zhXdJXTTCoG39QsrOCiOabnWj2jecouOqbchu3EfhtSHxIB5Uugnm9JaizenOy39h7ne3+fLikIjeW88+rgszw==
+  dependencies:
+    "@babel/runtime" "^7.23.2"
+
 i18next-parser@9.0.2:
   version "9.0.2"
   resolved "https://registry.yarnpkg.com/i18next-parser/-/i18next-parser-9.0.2.tgz#f9d627422d33c352967556c8724975d58f1f5a95"