There is a style of restaurants in Rio de Janeiro that perfectly fits the needs and difficulties of a foreign traveller: Kilo restaurants.
Instead of having to choose from a Portuguese written menue where most dishes have untranslatable proper names, how about checking out the food before putting it on your plate?
In Kilo restaurants you choose your food from a variety of buffets of salads, hot plates, meats and fishes, and even desserts. you will then put your filled plate onto a balance and pay per weight.
Kilo restaurants are a fast, cheap, fresh way to have lunch. The food is freshest during lunch time between 12.00 and 14.00. In the South Zone, especially Copacabana, you also can find kilo restaurants for dinner. Dinner time is somewhere between 19.00 and 21.30.
Another good lunch option are executive lunches, here, they are called Prato Feito, a prepared plate, and it features rice and beans and some kind of meat, chicken, or fish. As side dish you can usually choose between salad, Fench fries, farofa, or maionese. Prato Feito is often offered in small bars and in bistros, usually there is a sign advertising it.
Third good option for relatively fast, good but not too fancy food are the beach kiosques in Copacabana. Often they are run by bar or restaurant chains and offer food accordingly. There are various kiosques like Habibs, Rei do Camarão, Manuel&Joaquim, etc.