Do you mean the Calendar of JavaScript?
If so, this is not the issue of piCal, but the issue of XOOPS core.
piCal is only a user of 'include/calendarjs.php'.
The JavaScript uses the language in the part of XOOPS core.
Therefore, if you'd like to improve it, write it to
http://www.xoops.org/ or XOOPS French, please.