Kartoshka-kroshka is a street food chain that is everywhere and is pretty good when you are hungry, on a budget and need something fast. You get baked potato with butter and cheese and then can choose various toppings on the top of it. The toppings are on display, so you should be able to manage without understanding much Russian. Eat there or take away.
Map with locations is at http://www.kartoshka.com/cafe/find-cafe/, but you can just look there once to see their logo and then find them in the city as you walk.