Most ATMs in Rio are card selective, your ATM card may work in one machine and fail in another. The same applies to card payments at shops and restaurants.
The local currency in Brazil is the Brazilian real. If you're coming from South and North American countries you can change your money at forex centers, for other countries it might be difficult hence you'll have to change your cash to dollars then to reals later on in Brazil.
However that's a very risky alternative, you can just carry your ATM card and make sure you change your money at the airport. ATMs there don't easily reject foreign cards.
Most ATMs use Portuguese but you'll find some in English (There are many ATMs across the airport, however, they are found in clusters, out of every 3 machines you'll find at a certain corner within the airport, one probably uses English.)
Don't use ATMs at isolated ends of the airport, they're bound to be scammed.
If you're having a hard time locating an ATM that uses English ask the airport staff ( not just a random person) to help you find one. Don't ask them to help you use the Portuguese ATM in case you don't understand Portuguese, you can be scammed.