Hi Nizar,
So are you saying that none of the ext is showing in French? I'm guessing that you have already applied the French locale to that user and all of that is working fine. If so, can you try editing the SMTP Settings under the Notification Device for a user and see if you can get the french language to show up?
I know we have some system generated data that gets added to an SMTP notification that is in English only however it would be something related to message count.
I'll be doing some lab work myself as well to verify this.
Brad