This may be strange, but I personally love everything to be on English, while I need the front to be on Russian for our end users.
Actually, some of our staff will also prefer English language for back-end, but not everyone, so probably personal configuration for admins/staff in addition to global one would make sense, too.
Finally, because our country speaks both Russian and Ukrainian, I would like to make it possible for every forum user to be able to select which one fits better. And for this to work well, I’d be extremely happy to be able to select which languages are allowed to switch between for the end users (one setting) and for admins/staff (another setting).
If implemented like described above, I believe it will cover every possible scenario and preference.