Good news everybody, I've played around with SpamAssassin and must say that it works quite good for my own mail.
I've sorted about 1500 mails from the info-de-l archive and trained the SA's Bayesian-Filter on these mails only.
After de is the only one upto now with an open info address, I'd suggest to create account/mailing-lists for all the other languages, and let them train the Bayesian-Filter.
Using a global filter is possible but a very bad idea cuz we simply have to many languages. Any Chinese mail is spam for info-de-l but wouldn't be for info-zh-l.
I'll write a small tutorial and put it to meta as soon as it is back from the Northpole (or whereever our db is).
Best is to wait until a seperate mail-server is installed and user-accounts for every language-team can be handed out.
ciao, tom