I think the versions with "in" could be used in English, although much less commonly than "It's downtown." For instance, at the moment "That's in the downtown" brings to mind a situation between two people visiting a city where neither of them lives working out the location of something.
"It's at downtown" isn't idiomatic in my dialect of English at least; I don't think I'd ever use "at" with "downtown." "It's in a downtown" is possible if you don't what to specify which downtown "it" is in. I think that does start to diverge from the ordinary meaning of the Ukrainian sentence, but I'm certainly no expert in Ukrainian.