The official language of Serbia is Serbian, a south Slavic language used primarily in Serbia and Montenegro and Bosnia and Herzegovina. Serbian is almost identical to Croatian and very similar to Macedonian and Bulgarian.
In cities you will have no problems finding English speaking people to help you out, but this could be a problem in smaller towns and villages. It is therefore recommended to carry a dictionary or a phrase book with you.
The Serbian alphabet is very consistent: one letter per sound with an insignificant number of exceptions. This phonetic principle is represented in the saying: "Write as you speak and read as it is written", the principle used (though not invented) by Vuk Stefanovic Karadic when reforming the Cyrillic spelling of Serbian in the 19th century.
Another rare feature of Serbian language is the presence of two alphabets: Cyrillic and Latin. The two alphabets are almost equivalent; the only difference is in the glyphs used. This is due to historical reasons; Serbian once being a part of the Serbo-Croat unification brought Latinic usage into Serbia. In everyday life you will see both alphabets being used all the time, in newspapers, street signs, etc.
For details please check this Wikipedia entry
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Learn some basic Serbian phrases