Fix translation of French months

Official translation of French months (followed by glibc, GNOME and
so on). See http://fr.wikipedia.org/wiki/Mois.

For references:
- date widget Chrome in HTML5
- https://github.com/jquery/jquery-ui/blob/master/ui/i18n/datepicker-fr.js
- GNU coreutils, glib, GNOME, etc