...does it honestly bother you when a complete stranger who probably doesn't know the end of a pen from a penis tries to tell you your business? I think I understand a measure of your frustration, but be realistic; a review is only an opinion. It holds only as much power over yourself as you give it. Just like the one I'm about to give you!
Personally, I dislike the practice of author's notes. You're posting a story for readers to read. The content should stand alone and speak for itself. If there's a problem with the clarity of your content, then your reviews will usually reflect that. I don't think getting annoyed really serves any purpose. If the German is rotten then the German is rotten (not to be mistaken with me thinking Germans are rotten, I love sauerkraut as much as the next person...). End of story. If you know, then why does it matter?
Posting on fanfiction sites is often a hilarious practice, one I suppose that requires thick skin. Let's face it, this is the internet. There are a whole boatload of morons out there (I might be one of them!!! ). No sense getting worked up over them. You'll burn out.
And on one last note: have you considered taking the fic down until you've finished the rewrite? If reviews pointing out mushy areas of your fic you've already stated you're going to rewrite bother you to such an extent, why not bypass the situation completely and finish the rewrite before posting again? Or...why not remove the German 'for now' and put in a corrected version 'later'?
You can't stop people from voicing opinions. But you do have a measure of control over them. The latter, of course, is entirely dependent upon you, your writing, what you choose to post, and when you choose to post it.
Good Luck!