Actually that's not how you say it in Russian at all
Russian would be "Privyet kuzen" or "Zdravstvuitye kuzen"
edit: well i just looked up burazeru and can't even figure out what it means :p... in any case, zdravo isn't russian.
Also let me just note that in the game they mention Niko helped Bulgarin smuggle people into Italy. Croatia and Slovenia are the only former Yugoslav states that share a border with Italy, just putting that out there! But then why would a Croat say that his Serbian is not so good anymore (as Roman says near the beginning of the game)? Given the fact that he did partake in that business with Bulgarin, he probably spent time in Croatia anyway! When it comes down to it, he's just Yugoslavian... and I'm sure Rockstar left it ambiguous on purpose.
Edit: well I guess they could've smuggled people by boat, too. That would give Serbia and Montenegro a sea border with Italy. So whatever, it's just completely ambiguous then...