Solved it myself an added the solution to the Wiki
Just add a new line like
HTTP Accept Language=de;q=1.0,en;q=0.8
to the [Adv User Prefs]-Section of the opera.ini file (~/.opera/opera.ini)
Change language code "de" (=german) to match preferred language.
can i change operas accept language heade so i get the german version of documents instead of the english one? i can do this on my desktop easily, but theres no visible option for this in the maemo version....