Bulgarian hasn't started but the game is now compatible with! Translating the game from scratch is a long process but that could be very rewarding as it will increase your English skills!
If you want to translate it to Bulgarian, just ask for it on Transifex, start translating and I will add it to the language list in game in a future update!
For example, a Czech team of 2 people has started the translation from scratch two weeks ago.
EDIT: From a technical POV, we've allowed the game to display ANY character that your local Windows installation can. All the languages in the list of the first post are available because the encoding of characters in compatible with our method. Yesterday, i opened the game to last three languages compatibles (Chinese, Japanese and Korean).