What would be the point of having a UI in French (or, for that matter, in any human language other than English), when the site's content is almost exclusively in English?
When I am not on PM or some other English language site, I spend a sizable part of my free time contributing on French-language Perl forums, writing articles on Perl in French and also translating into French articles and documentation about Perl. This week I have spent quite some time reviewing a translation into French of the first chapters of chromatic's Modern Perl book. So I am actively involved in the promotion of Perl among French speakers.
But I would certainly not waste time translating the PM UI into French, because it would be just useless.