Forcing Russian language and culture upon conquered nations was and remains one of the main methods of extermination of conquered peoples. There's even a word for it, Russification.
Your question is the same as asking what was so bad about "speaking English" in the context of American native population and European settlers. Most of modern-day Russia is a land conquered from natives the same way European settlers conquered the Americas and wiped out native population using a wide range of tools from direct massacres to forced cultural assimilation.
The question is culturally as insensitive and uneducated as asking a black person in the Americas what's so bad about working on a plantation, or why they're not happy with sitting at the back of the bus - does it rock too much there or what's the problem?
If you want to educate yourself on how Eastern Europe suffered in the 20th century, then I recommend this book: https://www.amazon.com/dp/0465031471
whoosh