Is Serbia in the EU? I don’t think it is. Thus, if it isn’t, it’s language wouldn’t have to be an EU language in any case.
The European Union has 24 official and working languages: Bulgarian, Croatian, Czech, Danish, Dutch, English, Estonian, Finnish, French, German, Greek, Hungarian, Irish, Italian, Latvian, Lithuanian, Maltese, Polish, Portuguese, Romanian, Slovak, Slovene, Spanish and Swedish.
Notice, many minority languages - used within nations with other official languages - did not make the list: Rhaetio-Romansche, Basque, Welsh, Scotland’s Gaelic, Lapp, Livonian, Csango, Roma, Finnish, etc.