Harald, I did brought this up a while ago (Sept. 4), and there was a response from Ken Manheimer. He explained that because the core developers currently are trying to stablize a 1.0 release the issue of internationalization (i18n) is not currently on the priority list. I was about to point you to the archives, but couldn't find anything before Oct 2. Instead I'll send you a copy in private, don't want to duplicate it on this list.
Like you I am prepared to do a translation (in my case Swedish). I did try a translation of the human presentation text in the 1.0b5 release, but gave up after a while when I realized it got a little bit too complicated with too many modules involved. In order to achieve i18n of mailman, it definitely involves some architectural considerations and (at least) partial change of the code. There seem to exist ideas in the heads of the core developers on how to achieve i18n in mailman. But we might have to give them some time and bring this up again later.
Tomas