[BUGFIX] Follow up: Add the encoding for all mb_* functions

Follow up:
Add the encoding for all mb_* functions

Resolves: #54770
Change-Id: I25b9531c74fb862f02c7fe888c3d42cf5674880e
Reviewed-on: https://review.typo3.org/31196
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee