Be careful when nodding to mean "yes" and shaking your head when you mean "no" in Bulgaria--Bulgarians do it the opposite way! However, if they know they are speaking with a foreigner, they may accomodate by reversing the motions. Just be sure that it's clear what set of standards they are using (and which they think you are using!) when conducting everyday transactions. You don't want to agree to something you would rather refuse.
In Bulgarian, you say "da" (да) for yes and "ne" (не) for no. When in doubt, use these easy-to-remember words to make yourself understood.