🌐(frontend) add Content-Language on doc access endpoint

We send an internationalized email from
the POST /api/docs/:docId/access endpoint.
We add the language of the website with the
Content-Language header.

src/frontend/apps/e2e/__tests__/app-impress/doc-member-create.spec.ts

@@ -121,6 +121,9 @@ test.describe('Document create member', () => {
     ).toBeVisible();
     const responseAddUser = await responsePromiseAddUser;
     expect(responseAddUser.ok()).toBeTruthy();
+    expect(responseAddUser.request().headers()['content-language']).toBe(
+      'en-us',
+    );
   });
 
   test('it try to add twice the same user', async ({ page, browserName }) => {