In your language file, (/YOURSITE/language/de-DE/de-DE.com_comment.ini) just open it up in some editor (Notepad is great for this!), and just resave it as UTF-8 format.
Then just upload it to the same spot... and that should fix it...
PS, not sure if German is de-DE, but I hope you understand the process...
OPS, I just read your post again, that you get a different response regarding frontend editing to backend...
This sounds like a collation problem that stems from the SQL... at least I think... hehe, Daniel knows that stuff... If my previous suggestion doesn't work...