Not it's not a bug of ZCS. It's my bug ;-)
I set the right language on the user side and it works.
However, there still are standard messages to translate for example the notification message : "
New message received at
my_account@testserver.xxx.
Sender: xxxxxxxxxxx
Subject: test notification en francais"
When i'll have an access to the system i'll have a try modifying the .properties files.
Thanks