Even Russians themselves reflected on this, noting that most volunteer forces they have faced so far come not from the Western (majority-Ukrainian-speaking) regions of Ukraine, but rather from Kharkiv, Dnipro etc. Their explanation is that all those people are "brainwashed by Ukrainian Nazis".
There is some real contention with the status of Russian as 1) the official government language, and 2) the official education language in schools. But I don't think it's fair to phrase that as "banning Russian".