[BUGFIX] Context-independent access to lang object

The connector services may be called in the FE or BE context.
The base service should thus provide a context-independent way
of accessing the language and charset converted instances
that reside in different "places" depending on the context.

Resolves: #55361
Releases: 2.4
Change-Id: I63463a480c6a47207142162af764b7940a682e7a
Reviewed-on: https://review.typo3.org/29096
Reviewed-by: Francois Suter
Tested-by: Francois Suter