I strongly disagree with keeping non-English text in the body after translating. It invites other users to reply in non-English as well. And it's very simple: this is an English-only site. Also, no need to keep a copy of the original; the original text is never lost and we can just view/link to the original revision.

So, my proposal would be:

• Get yourself a Google translation of the post.

• Edit the post with the English translation only.

• Include a note at the top: Original revision was in German (with a link to revision #1)

  This note is an invitation for German natives to improve the translation. Optionally mention it on chat and ping users you know to be able to improve the translation.

I strongly disagree with keeping non-English text in the body after translating. It invites other users to comment/answer in non-English as well. And it's very simple: this is an English-only site and we can expect users to obey this. 

Also, no need to keep a copy of the original; the original text is never lost and we can just view/link to the original revision.

So, my proposal would be:

• Get yourself a nice automated translation of the post.
• Edit the post to contain the English translation only.
• Do your own editing on it like any other (poor-English) post.
• Include a note at the top: This is a translation of the original post in German (linking to the non-English revision)
• Have the translation being edited like any other (lower) quality posts by the community.

This note on top of the question/answer is an invitation for native tongues to improve the translation (including the OP).