Obviously I have no advice regarding actual practical ways to get involved and practice the language in Sofia, however in my experience with language it has definitely proven true that the only way to REALLY learn it is total immersion where English just isn't an option. It's very, very hard to immerse oneself when you have a lot of English-speaking friends and colleagues (and for you, students!), but if you are open to it, make a pact with your friends who speak English that you are only going ot speak Bulgarian to one another, or to your Bulgarian-speaking friends, that they will speak Bulgarian to you as much as possible and not let you off the hook!  Good luck! It's a brave thing that you're doing! I think you will see some real results! :)