BuddyPress.org

I have a bbpress/buddypress website in French & German, translated with WPML.
When a user registers himself on the German version, he gets German emails. When a user register himself on the French version, he gets French emails.

When a German speaking user is asking for a friendship with a french user, the french user gets the mail notification in German, instead of French.

Possible solution: I add a field in the registration form where the user can choose his language BUT how can i synchronize this with the mail notifications the user gets?

Thanks a lot for your help!