Russian language was used as a way to communicate with other people. Like english today, a way I can communicate with someone from Italy as example. I remember Lenin writing about the question, what will be the "common language". He came to the conclusion, that it would develop naturally through trade. Learning Russian became mandatory later.

My father, ethnic Russian, learned Kazakh language in school as secondary language. He can understand Ukrainian and Belarus. Not speak, but understand quite well. However I cant understand it, probably because I start to forget more and more my own mother tounge.

Today english is also preferable to learn in Russia.

Don't understand your downvotes