There’s ratified UN conventions on what is legally considered genocide…
I would suggest that instead of sourcing your understanding of genocide from Lemmy comments, you go read it, or at least it’s Wikipedia entry.
Also yes, cultural erasure can be an act of genocide, but I doubt the Belarus situation would quality at the moment. Given their governments participation in the assimilation, it probably requires some additional actions, or metrics. But, it’s not like I’m a human rights lawyer, so maybe I’m wrong.