In a realistic movie they'd speak Russian/Ukrainian. I think wrong accents in that case would be ok because only Ukrainians would notice (it won't air in russia I'm guessing).

But Netflix probably wanted well known actors instead, and know lots of people dislike reading subtitles, so chose to use english, which is understandable in some ways. Myself I find documentary(-style) movies in the original language to be much more believable, and reading subtitles is a price well worth paying for e.g. "Der Untergang" and similar.