posted 11 years ago
There is no official listing as far as I know, but all big languages are available. I set it myself to German, Dutch, French, Portuguese, Spanish, Japanese and Italian successfully already.
From the Google documentation:
Google's servers support many languages for voice input, with more arriving regularly. You can use the ACTION_GET_LANGUAGE_DETAILS broadcast intent to query for the list of supported languages.
Since languages are added from time to time, an official list would definitely need continuous maintenance too.