Even though the UserLanguage service has been migrated to the new service container, it was decided in T278819 that this service should be deprecated and gradually retired out of usage, because services in the service container should be independent of a specific request context, including the user language for that request, and $wgUser will be deprecated eventually (T159299).
The method was deprecated in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/676422
Currently, this method is called in the following extensions:
In each of these extensions, an alternative way to obtain the user language should be found to replace calls to this method, either by utilizing the RequestContext or otherwise (in places where the request is not available).
- WikibaseRepo::getUserLanguage is no longer called from extensions that we own