✨(backend) add API access for 'language' attribute on User model

- allow the language attribute on the user to be updated via API
- add frontend function to update the user language via API
- extend defaults on the test users, to have fixed language in E2E tests
- extend types and variables using the types with the new field

src/backend/core/tests/test_api_users.py

@@ -158,6 +158,7 @@ def test_api_users_retrieve_me_authenticated():
         "id": str(user.id),
         "email": user.email,
         "full_name": user.full_name,
+        "language": user.language,
         "short_name": user.short_name,
     }