The language filter is the easiest way to mark an entire subset
of message types as spam: filter out those messages in foreign languages
you get unreadable spam from.
The language filter does a statistical analysis of the language
the message is written in. This does work more or less quite well,
but it has a good chance to fail for short messages and for 'mixed'
languages (imagine a german message with some program language source
examples parts). It works good for languages with different char set
like Japan or Russian.
The test creates a relative high load (-> needs its time, no
no matter how many languages are selected.