> I also highly doubt USSR would accommodate people who move in and don't bother to integrate into the culture and speak Russian

USSR was schizophrenic in that regard. Early on, it had https://en.wikipedia.org/wiki/Korenizatsiia, which was quite literally about de-russifying the local elites and promoting local languages. Then Stalin did a hard 180 on that, like he did on so many other things (e.g. re-criminalizing abortion and homosexuality, or reintroducing paid high school). I don't think it's unreasonable to argue that under Stalin, the USSR truly switched from being a left totalitarian dictatorship to right totalitarian.