Yes you can, you just have to be around for the finals in October in case you win the preliminary round:
DCM FAQ (in German)
In case you're interested in participating in the DCM or in the regular cosplay competition, just tell them upon registration that you can't speak German. There's a couple of Dutch cosplayers who are regulars at AnimagiC and who actually won the cosplay competition at least once (don't remember their user names right now) so it definitely shouldn't be a problem.
The DCM actually has an English version of their rules (at least of the 2008 version) that I'm sure they could send you if you're interested in learning more about the competition.