Thank you Chronon for the link. I could have thought about the wiki myself.
Stupid me, I confused .lang with .lng files. But anyway I was unable to cope with the explanations on the wiki. With the the help of a friend I could patch the deutsch.lang file with my changes. I have no idea what he has done, but now I have a complete german translation of rockbox on my clipv2.
I will submit the deutsch.diff and the deutsch.lang files in the tracker. See:
http://www.rockbox.org/tracker/task/12433I hope that I have contributed a little bit to this wonderful project.
Greetings,
c_k