Andrea Medici reported that he uses Spambayes with Italian. He posted his configuration file. Differences included reducing the number of discriminators to 50 (default 150) and changing the ham/spam cutoffs to 0.2 and 0.25 (!), respectively (defaults 0.2 and 0.9).
Just setting ham/spam cutoffs to 0.2 and 0.4 seems to be effective with a recent load of German porn mails.