I'm Serbian and in the game Niko and Roman speak Serbian with a terrible accent, which makes sense since both voice actors are American. They sound exactly like how i would imagine a non-native trying to speak it.
As Russian, I perceive the discussions about Russian origins of Niko as at least strange. It is obvious because of his name, surname, remembering of Yugoslavia's war. He is from Serbia, or to say precisely, from Yugoslavia...
Don't forget in TLAD, when Billy told The Lost MC about Jason's death, he mentioned that the person who killed was probably a Serb, which he was referring to Niko, as Johnny also remembers it when Billy changed his mind by gaslighting that The Angels of Death killed Jason
Niko, Roman, and Darko all speak Serbian in the game. (though it's pretty terrible in terms of translation and delivery from what I've heard from Serbians IRL)
There's also a hint where you have to deal with Eddie Lowe and Eddie asking him where is he from because it sound like he's not from America and he was naming places but Nico never said he was rushing he just told him he's a smart guy
Before Dan Houser revealed of Niko's Serb origin, there were always speculations about what is exacty his Balcans origin. It remind me of Second Jugoslavian war in comments about Niko's origin.
Blame the game itself, and also the stereotypes surrounding it! In that Russian clothing store, you can buy a traditional "ushanka" with a Russian flag on it, which is humorously picked by a lot of players for the laugh. Why is there such a choice in general? Why not any Serbia related clothing? And people are surprised that Niko is called Russian a lot of times. Second thing is the accent, which sometimes sounds "too Russian" in some scenarious, as Michael Hollick spoke his Serbian lines with an exaggerated accent, which is confirmed even by Serbians itself. And since I know the language as well, I can confirm it too, along with some obvious "out of translators" abominations.
In Revenge ending, Rascalov refers to Niko as a “Baltic”. This is obviously a mistake, but it would be great to see Baltic people represented in the media :(