I wouldn't agree that hints from 2011 or anything linked to Prestashop releases < 1.7.0 would be helpful. Prestashop 1.7 stores the translation items for (in this case German) mails in
/app/Resources/translations/de-DE/EmailsBody.de-DE.xlf
/app/Resources/translations/de-DE/EmailsSubject.de-DE.xlf
Or hopefully most of them, because in 1.7 you cannot always be sure.
Those of you who run a version < 1.7 and have some skills in German find everything they need to find translations in a topic moved from this forum to here.
But in any case I'd suggest to first use the translation section in your shop's back office.