Serbian and Russian languages are close to each other but they are different. For example, Russian alphabet doesn’t have letters Ј, Љ, Њ, Ђ, Ћ and Џ. So TTS engine cannot pronounce Serbian names if they contain these letters and TTS uses Russian language. I have met these situation with OsmAnd yesterday — it uses default (Google?) TTS and lost significant part of names like Ђурђа.