The jQueryUI datepicker is supposed to be loading localized strings from the config file, calendar.inc, but it isn't. Even with a correctly-translated copy of calendar.inc English strings are shown.
Correction: The French version of the configuration file has an error because one of the translated words has a single quote: currentText: _('Aujourd'hui'), Converting the surrounding single quotes to double quotes should fix the problem.
*** This bug has been marked as a duplicate of bug 8942 ***