In several areas of Serbia, there are significant percentages of people who are not Serbs. Apart from the known example of Albanians in Kosovo & Metohija, the other area with mixed national composition is Vojvodina (everything north of rivers Sava and Dunav (Danube)), with Hungarians, Romanians, Slovaks, Rusins (there are actually only two villages of them), remains of Germans (though they have no separate villages of their own anymore), and then a smattering of single-percentage populations of Gypsies, Ukrainians, Albanians, Croats.
The statute of Vojvodina is thus written in five languages (Serb/Croat, Hungarian, Romanian, Slovakian and Rusin). The official stamps on documents may be up to 10cm in diameter.
All these people are citizens of Serbia, but their nationality is something else. This is to contrast the American usage of nationality as citizenship, and ethnic origin as nationality. Since most of Vojvodina (literally, "Duke's") was initially in swamps, it was populated over several centuries, as irigation progressed, by whichever people were sent at the time - the South German poor, the Serbian mercenaries, the Hungarian peasants without land etc etc. They would just arrive in a caravan to their allotted land, and build a village. They'd keep their cultural identity for generations, even in villages where they had two peoples living next to each other. There are many pairs of villages, usually one Serbian and the other Hungarian or Romanian or Slovakian. It is quite common that people in such mixed places speak both languages; there were (and maybe still are, in some places) bilingual schools. In other places, part of the school learns in one language, the rest in other. A place may have two or more names, one in each language.