feat(seo): redirect / to /fr/ or /en/ server-side based on Accept-Language

src/app.js

@@ -68,6 +68,13 @@ export function createApp(config, prisma) {
   app.set('trust proxy', '1');
   app.use(compression());
 
+  app.get('/', (req, res) => {
+    const acceptLanguage = req.headers['accept-language'] ?? '';
+    const primaryTag = acceptLanguage.split(',')[0]?.split(';')[0]?.split('-')[0]?.trim().toLowerCase();
+    const lang = primaryTag === 'en' ? 'en' : 'fr';
+    res.redirect(301, `/${lang}/`);
+  });
+
   const storage = multer.diskStorage({
     destination: (req, file, cb) => cb(null, path.join(config.storageDir, 'uploads')),
     filename: (req, file, cb) => {

test/staticServing.test.js

@@ -39,6 +39,26 @@ afterAll(async () => {
   await fs.rm(frontendDist, { recursive: true, force: true });
 });
 
+describe('root redirect', () => {
+  it('redirects to /en/ when Accept-Language prefers English', async () => {
+    const response = await request(app).get('/').set('Accept-Language', 'en-US,en;q=0.9');
+    expect(response.status).toBe(301);
+    expect(response.headers.location).toBe('/en/');
+  });
+
+  it('redirects to /fr/ when Accept-Language prefers French', async () => {
+    const response = await request(app).get('/').set('Accept-Language', 'fr-FR,fr;q=0.9,en;q=0.5');
+    expect(response.status).toBe(301);
+    expect(response.headers.location).toBe('/fr/');
+  });
+
+  it('defaults to /fr/ when Accept-Language is absent', async () => {
+    const response = await request(app).get('/');
+    expect(response.status).toBe(301);
+    expect(response.headers.location).toBe('/fr/');
+  });
+});
+
 describe('static asset serving', () => {
   it('sends HTML pages with Cache-Control: no-cache', async () => {
     const response = await request(app).get('/fr/');