In Portuguese, "Chill out" can be translated as "Relaxa" or "Fica tranquilo" (informal) or "Relaxe" or "Fique tranquilo" (formal). Both phrases convey the idea of asking someone to calm down or to relax in a laid-back manner. Use the informal version with friends or people you are on familiar terms with, and the formal version with people you don't know well or in more professional settings.