fix(app/membership.py): Set user's preferred language
For a newly created user with no preferred language, the preferred language would be then, set as the preferred language of the mailing list s/he subscribes to.
This fixes #608 (closed)
For a newly created user with no preferred language, the preferred language would be then, set as the preferred language of the mailing list s/he subscribes to.
This fixes #608 (closed)