Right now, Google Translator Toolkit supports the following source languages: Arabic, Belarusian, Bulgarian, Chinese, Croatian, Czech, Danish, Dutch, English, English (UK), Finnish, French, German, Greek, Hebrew, Hindi, Hungarian, Italian, Japanese, Korean, Norwegian, Polish, Portuguese (Brazil), Portuguese (Portugal), Romanian, Russian, Spanish, Swedish, Thai, Ukrainian, and Yiddish.
The product is a work in progress and we're continually adding functions and features. We've heard from users that more source languages is something we should make a high priority, so we've done that and we hope to launch more languages soon.