✨(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/demo/management/commands/create_demo.py

@@ -179,7 +179,8 @@ def create_demo(stdout):
                     is_superuser=False,
                     is_active=True,
                     is_staff=False,
-                    language=random.choice(settings.LANGUAGES)[0],
+                    language=dev_user["language"]
+                    or random.choice(settings.LANGUAGES)[0],
                 )
             )