@Evan,
The only sensible option I see is to have separate courses for those two populations of students. Then it's quite easy to "force" the language in each separate course. You would thus have everything (including the interface language) in English for the English-speaking students and everything in Russian for the Russian-speaking ones.
Other than that I cannot see the benefit of having one unique course for both populations. Any reasons we do not know of? What is the subject matter being taught in those courses? Languages or ... ?
Joseph