Via Google Blogoscoped post I learned it is possible to search for messages in Gmail based on what language they are written in. The operator is called “lang” and can be used like so:  “lang:ru“ or “lang:russian“. The operator can be used both in regular searches and in filter conditions.  As noted in the comments, this might be useful for sorting out spam messages (label with “Spam“) written in languages that you don’t understand (Chinese, for example, – “lang:zh“).
For me personally, this comes very useful, since most of my friends and family (at least those with who I communicate via email) speak both Russian and English, and sometimes it takes too much time going through all the messages instead of picking just those in one language (for those cases when I remember the language).