Catherine II banned Ukrainian in the part of the Ukraine that fell under the old Russian Empire. This is why Eastern and Southern Ukrainians are so pro-Russia, despite being ethnically the same as Western Ukrainians.
Language causes major issues when it comes to identity and national cohesion.
That's why I don't like the accommodation the Somali government is giving to Af-Maay by recognizing it as part of the Somali language, promoting its usage, allowing for it to be used as a medium of instruction in primary schools etc. It should be unrecognized and not supported by the state, IMO. Sounds harsh, but long term for the better.
Belarus also got its own language distinct from Russian, but unlike Ukrainian it is dying out and they are all pretty much Russophones. They don't even use Belarusian at home (less than 15% do) and look at their great loyalty to Russia.