Default to language from request in L10N\Factory->getUserLanguage()

Try to get the language from request before falling back to the instance
default.

This fixes the detected user language during first login of a new user
and ensures that the returned language is consistent during the login.

Also partially fixes a bug with Collectives folder being initialized
in different languages at first login, see
https://gitlab.com/collectivecloud/collectives/-/issues/238



Signed-off-by: Jonas Meurer <jonas@freesources.org>