Filter Foreign Language Spam with SpamAssassin

Today, March 25, is the birthday of Béla Bartók. His string quartets are written in the language of music, but the language in which he wrote his letters was Hungarian. Magyar is just one of the languages that SpamAssassin can filter or allow, as you like it.
If you never get good mail in a language you don't speak, here's how to make SpamAssassin look for foreign language spam and filter it away.