This is already possible. You aren't required to use the same username on all sites; you can use different usernames on different sites.
Simply enter a different username on a given site's profile editor, and click the translation of "Save changes just for this community". You'll have a different username for that community.
I understand that this is a bit of an issue for the Russian language specifically, since there are two sites in the Russian language (ru.SO and rus.SE; not counting russian.SE since it's technically not a localized site), but that's a bit of an edge case since there's generally only one localized site per language and this is unlikely to change. (This was a bit of a special case since the two sites were originally operated by a different company and became SE sites after SE Inc. acquired that company.)