Also struggled with the English form of Weißrussisch and missed Macedonia when browsing through former YU.
Anyway, Slava! :)
If you wish to avoid adding those languages, I suggest you set a more specific criterion for inclusion. For example, you could specify that only languages with 1,000,000+ speakers qualify, or that only official or national languages of countries qualify.
Indeed, some debate exists about whether the said languages are actually languages or dialects, but for that matter, the same applies for the varieties of Serbo-Croatian and for Macedonian too.
The quizmaker is kind enough to provide a source. Wikipedia agrees with him. If you feel the need to argue the position, you could at least try to make the effort of presenting an argument. Even if it's a feeble one.