I would create only one arrival locale and decide which one is the default, probably the de_de would be the formal and the new language weighs be for the informal.
As for the error messages I would just copy over the po for from django
--
You received this message because you are subscribed to the Google Groups "Django users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to django-users...@googlegroups.com.
To post to this group, send email to django...@googlegroups.com.
Visit this group at http://groups.google.com/group/django-users.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/20150729115805.4e53c25f%40kvothe.
For more options, visit https://groups.google.com/d/optout.