It's not a rule. Sounds like you ran into an airline employee who didn't understand the regs. Just proficiency in Portuguese required (to be able to understand and relay emergency instructions), but even that is not regularly enforced. I've sat in the exit row and I'm not a Brazilian citizen. I have a permanent residency card, but have never been asked to show it. At what point were you asked your citizenship status? At the airport when you wanted to purchase the upgrade?

I've seen passengers get up and move to the exit rows (sometimes empty because people didn't want to pay the extra R$) once a flight has commenced. No one asked them their citizenship or what language they spoke.