Seeing how the site is dedicated to the Russian language, I think it needs to be be usable by the majority of the target audience, that is the people who speak Russian. Of course, we can't forget about the people who are just starting out with the language or are merely interested. Should we be allowing question in languages besides Russian and English, and if we do, do we need to make any special provisions for them?
As far as I know, the existing sites for languages use both English and the language they are devoted to. So in our case this means that it should be allowed to ask questions in both Russian and English.
I think this is fair, because not every Russian-speaking person knows English well, but they shouldn't be deprived of the opportunity to ask questions about their native language. The only alternative is a site in Russian about Russian, but it just seems awkward to me.
This is site dedicated to Russian language, so questions in Russian language should be allowed, as analogy to German Language and Usage, where most of the questions are asked in English, but some are asked in German and this is widely accepted.
And about interface, I haven't seen the Stack Exchange site having bilingual interface, but in my opinion at least FAQ could be translated (both language versions could be on the same page, what would not require any change in interface).
On the Spanish Language & Usage site an effort is made to add an English translation to posts made in Spanish, and to add a Spanish translation to posts made in English.
This applies to both Questions and Answers.
The sections each have a heading to state which language they're in (though it's always obvious). And the two sections are separated by three hyphens in the Stack Exchange formatting:
The result is a horizontal line like the one above.
I suggest we should encourage using the Russian language as the main one for this QnA, because it's all about Russian at the first place. We also should accept questions in any language at the best of our knowledge, but the answer should be solely in Russian for the sake of Russian, this QnA, and the person who asks their question.