Garett Rogers writes about an interesting hidden feature in Gmail which will help you find messages written in certain language. Try searching for “language:<language name>”. For instance, if you want to search for messages written in Spanish then search for language:Spanish.
It works with most languages, but oddly English isn’t supported. Otherwise, we could have created a filter which eliminates all unreadable mails written in different language. BTW, why was Google shy to inform us about this? Maybe they are still working on adding more search filters.

wrote, on November 18th, 2007
Definitely. The English language is not supported right now, right ? So, its probably evidence that they’re not finished. I think Gmail needs to grow up a lot both in design and in helping laymen use advanced filters.
wrote, on November 18th, 2007
English works well, but for many mails the language can’t be identified so that a search language:French leaves out some results
wrote, on November 19th, 2007
But does this feature works for Indian languages?
wrote, on November 19th, 2007
Shiva: Nope, they don’t support Indian Languages right now, but soon they should!
wrote, on November 22nd, 2007
I’m wondering if there will be also smaller smaller languages support. For example Slovene language (and many European languages). No sign yet.
wrote, on November 23rd, 2007
gmail’s current design is gud enough….i haven’t seen a better place for ajax than gmail…