✨(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/api/serializers.py

@@ -23,7 +23,7 @@ class UserSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = models.User
-        fields = ["id", "email", "full_name", "short_name"]
+        fields = ["id", "email", "full_name", "short_name", "language"]
         read_only_fields = ["id", "email", "full_name", "short_name"]