Are you sure?
In my experience once people are on facebook groups conversations tend to dissipate, same with people. With Edgeryders we strive to maintain an online space that already has better data protection and information accessibility than a commercial platform, among other advantages (if you haven’t, here’s our answer to the question of why we’re not using Facebook, asked before).
Exhibit A is this Georgian group with was accidentally set up while we were working there in a project similar to yours. What happened it not only drew some people away from Edgeryders, it had suddenly no goal to exist since there wasn’t any curated content, pointed questions to get people to really talk to one another instead of just sharing links once in a while (talk at each other).
I see you already have posts in this group in Russian and know how to use the Translate feature, why not release calls on social media that are in Russian and pointing to a post in Russian, would it not be enough of an encouragement for your fellow nationals to answer in the same language? I know, platform navigation and menus are still in English, but it did seem to work with Armenians here, at least getting people to come up with a first contribution. Then if you do community management in Russian, that should also help.
Just some ideas, perhaps I don’t know the full scope of your project…