Catherine initially invited any Western Europeans who wanted to go there, including British, French, Dutch, and so on. It's that only the Germans came in large numbers, as they had the biggest excess population and the most desire (other Western Europeans looking to immigrate chose the Americas first). Western Europeans were seen as closer kin to the Russians than the Mongols and Tatars they were trying to drive out.
My point is that by that time, Russia already owned the territory. I'm looking for alternate colonizing powers other than Russia, not alternate settlers within Russia.