If your language is not currently supported on Windows Phone, the Windows Phone Suggestion Box is a handy site to make your voice heard and vote up suggestions from other users. Once an idea has been implemented, you'll see a small COMPLETED label.
Interesting. But I guess my question is similar to the one before - I'm struggling to find out which languages are available in the UI already and which aren't. Is there a relaible list anywhere? It doesn't seem to be included on any list of features, bizarrely, given how important this is to many people.