Hi:
I created a French date Zimlet that works very well together with the English one. Search this forum or go to
http://files.whitehatmail.fr/ and login with "invite/invite". There you will find a Zimbra directory and a file zcs-fr-4.0.3_GA_406.tgz that installs the French translation, the French spelling and the French date zimlet.
From the architecture point of view, I think the best is to have separate zimlets and play with zimlets priorities to make they work properly.
In the case of the date zimlet, the FR zimlet has priority over the EN zimlet, so if a date is like 05/08/06 it will recognize it as Aug 5th (FR) and not May 8th (EN) (the priority is managed by the installation script I wrote).
You can also have different zimlets in differents COS, what gives you even more flexibility (you can have a DE+EN COS, plus a FR+EN COS for instance).
Carlos