🐛(frontend) fix browser language detection

The `Default language` E2E test detected the browser language was not
automatically detected when user does not have any cookie, it was always
falling back on the defaut language (en).

src/frontend/apps/desk/package.json

@@ -21,6 +21,7 @@
     "@openfun/cunningham-react": "3.0.0",
     "@tanstack/react-query": "5.72.1",
     "i18next": "24.2.3",
+    "i18next-browser-languagedetector": "8.1.0",
     "lodash": "4.17.21",
     "luxon": "3.6.1",
     "next": "15.2.5",

src/frontend/apps/desk/src/i18n/initI18n.ts

@@ -1,4 +1,5 @@
 import i18n from 'i18next';
+import LanguageDetector from 'i18next-browser-languagedetector';
 import { initReactI18next } from 'react-i18next';
 
 import {
@@ -8,7 +9,11 @@ import {
 } from './conf';
 import resources from './translations.json';
 
+export const availableFrontendLanguages: readonly string[] =
+  Object.keys(resources);
+
 i18n
+  .use(LanguageDetector)
   .use(initReactI18next)
   .init({
     resources,
@@ -23,7 +28,7 @@ i18n
     interpolation: {
       escapeValue: false,
     },
-    preload: Object.keys(LANGUAGES_ALLOWED),
+    preload: availableFrontendLanguages,
     nsSeparator: false,
     keySeparator: false,
   })

src/frontend/yarn.lock

@@ -944,6 +944,11 @@
   dependencies:
     regenerator-runtime "^0.14.0"
 
+"@babel/runtime@^7.23.2":
+  version "7.27.1"
+  resolved "https://registry.yarnpkg.com/@babel/runtime/-/runtime-7.27.1.tgz#9fce313d12c9a77507f264de74626e87fd0dc541"
+  integrity sha512-1x3D2xEk2fRo3PAhwQwu5UubzgiVWSXTBfWpVd2Mx2AzRqJuDJCsgaDVZ7HB5iGzDW1Hl1sWN2mFyKjmR9uAog==
+
 "@babel/template@^7.25.9", "@babel/template@^7.26.9", "@babel/template@^7.27.0", "@babel/template@^7.3.3":
   version "7.27.0"
   resolved "https://registry.yarnpkg.com/@babel/template/-/template-7.27.0.tgz#b253e5406cc1df1c57dcd18f11760c2dbf40c0b4"
@@ -6627,6 +6632,13 @@ 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@8.1.0:
+  version "8.1.0"
+  resolved "https://registry.yarnpkg.com/i18next-browser-languagedetector/-/i18next-browser-languagedetector-8.1.0.tgz#e7fcc1084e9ca8fece329c8c05b000a6e25bd82e"
+  integrity sha512-mHZxNx1Lq09xt5kCauZ/4bsXOEA2pfpwSoU11/QTJB+pD94iONFwp+ohqi///PwiFvjFOxe1akYCdHyFo1ng5Q==
+  dependencies:
+    "@babel/runtime" "^7.23.2"
+
 i18next-parser@9.3.0:
   version "9.3.0"
   resolved "https://registry.yarnpkg.com/i18next-parser/-/i18next-parser-9.3.0.tgz#65c226cac54cd2783b59715a366be1e3515dd8e6"