The error messages in contact form are hardcoded in current version, but they are in language files already and thus will be available in the next release.
Sorry, do you mean the contact-form itself? That should be in German if you have a complete German language file. Try going to admin -> languages -> Edit the "German" language file. Check at the bottom that all items are in German, and not some items in English. If still fails, please provide link to your gallery.