You can't here. Itch.io does not have a feature for that.
There would be nothing stopping someone from North America buying the cheaper version and you might antagonise them when they realise they're being charged more. (Remember, your projects are linked, they will see all your projects by clicking on your profile.)
Probably the only way you'd get away with it at Itch.io, would be to release two separate games: one in Portuguese only and one in English only.