Bulgarian is a southern Slavic language, mutually intelligible with Macedonian and closely related to Serbo-Croatian, Slovene and Russian. If you know any of these (or another Slavic language) you shouldn't have much problem getting by. Ancient Bulgarian (also known as Church Slavonic) is considered the "Latin" or mother language of the Slavs. Some words and/or phrases might even be understood by Westerners since Bulgarian has a number of loans from other languages (most notably French, German, Turkish, Italian and increasingly English).
Modern Bulgarian is difficult to westerners, as it has three genders, the infintive has fallen virtually out of use, and articles are appended to the end of either the noun (if no attribute is present) or the first attribute (example: kuche = dog, kucheto = the dog, dobro kuche = good dog, dobroto kuche = the good dog), functioning almost like an agglutinative language. Even though the language has lost the case system entirely, Bulgarian makes up for the difficulty with a complicated verbal and noun system (A verb in Bulgarian can have up to 3,000 forms!). It takes a short while getting used to the Cyrillic alphabet, a writing system of which Bulgarians are proud. Be sure to be in Bulgaria for the celebrations of the "Den na Pismenostta" ("Day of the Literacy"). The Russian/East Slavic version of the alphabet is almost identical to the Bulgarian one.
Turkish is the second most widespread natively spoken language in Bulgaria, and it is generally spoken in areas populated by Bulgarians of Turkish descent.
It is also important to remember the fact that many Bulgarians - contrary to most nationalities - shake their head for Yes and nod for No! It is better to rely on the words da for yes and ne for no than on head movements. Bulgarians often use ciao for good-bye (instead of "Dovijdane") and merci for thank you (instead of "Blagodarya").
Most young Bulgarians have at least a basic knowledge of English or/and a second foreign language (usually Russian, but German, French or Spanish can also be spoken) and will often even take up a third one. Those born before the mid-1970s are most likely to speak Russian, German (because of ties with East Germany) or/and Serbo-Croatian and usually have limited or zero knowledge of English at all.