Yeah, it's impossible to be sure, this being Mollo, choir could be US singers. However I decided to add Chaga anyway because both him and Mollo were regular arrangers for some of John Powell's soundtracks and Chaga besides the expected arranger role also has Russian translation credits (or could be just transliteration) for one such (animation) album that I know of (Happy Feet Two).
So I think the non-Russian choir tries to sing in Russian
even for just a single track.