FW: return language for format-date

Perhaps a dumb question previously answered, but I didn't see it on the list
archives.  Apologies if I'm wrong.
 
date:month-abbreviation(), date:month-name(), date:day-name() and
date:day-abbreviation() are documented to return English.
 
date:format-date() does not specify whether it returns English or the local
language of the machine running XSLT for "MMM", "MMMM", "ddd" or "dddd".

Is date:format-date() meant to return English or Local Language in these
cases?

Regards,

Jonathan