To be honest, I'd just say Letters (which is the literal - and right, although may sound strange - translation) and if they didn't undestand, I'd explain the course.
Or you can say "I'm majoring in English and Portuguese", maybe. If literature is included in your course (it is, right?) maybe even "I'm majoring in English, Portuguese and Literature."
edit: the course of Letters also exist in the US, take a look: http://www.wesleyan.edu/col/
but af far as I could see in the use of this term, it usually refers to a college with Letter-related courses as Literature and Languages, but I may be wrong about that.