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