In ParserFunctions.php there is
wfMsg( 'pfunc_max_ifexist_category' )
but it should be
wfMsgForContent( 'pfunc_max_ifexist_category' )
If you now just change your user interface language, you see the category in that language.
With wfMsgForContent you'll see it in the normal content language.
Version: unspecified
Severity: enhancement