We recently upgraded Keycloak from 3.4.3 to 12.0.2 and since then the users locale is not set anymore during registration.
Previously the locale in the user profile was set to the locale used to display the registration page.
I attached a debugger to a local 12.0.2 keycloak and if you look at the UserLocaleUpdateAction the user locale is determined based on the authnote locale_user_requested, but this is not set when using the openid-connect registration url (which is created by the keycloak.js register function):
http://localhost/auth/realms/<realm>/protocol/openid-connect/registrations?client_id=<clientid>&redirect_uri=http%3A%2F%2Flocalhost%3A4200%2F&state=4917895c-ef91-403c-98ee-49f4025c3018&response_mode=fragment&response_type=code&scope=openid&nonce=df3574be-ddfc-4d42-8f88-bfc3094e29ff&ui_locales=de&kc_locale=de
If I go through the login page and click register, I eventually end up on http://localhost/auth/realms/<realm>/login-actions/registration?client_id=<clientid>...., from here the locale is set, but I can't seem to access this url directly.
I'm not sure if it is a bug with Keycloak or am I missing some new configuration?