How many languages do you speak? If you are like most people, probably only one or two. If you are fluent in multiple, you may find it useful to keep messages written certain languages separate -- or you may find it useful to create a filter that deletes messages in languages you don't understand. One of my readers stumbled across a new way to search through his Gmail for messages composed in Spanish, and I have confirmed that the same technique works for most other languages too.
Unfortunately, English isn't one of the supported search phrases, so I am unable to create a filter to catch foreign junk mail, but this simple search in Gmail shows you messages it believes is written in Spanish:
What would be even better is if you could filter messages by origin. I really only care about email that comes from a few different countries, and considering most spam originates in places I haven't even heard of, I wouldn't mind a way to filter those out too.