🐛(i18n) same frontend and backend language using shared cookies

frontend: switch to cookie-based language selection
backend: use cookie for language

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.');
+  });
 });