Charles Dance was specifically told to use his natural English accent even though he plays the US president in the film. The directors (in Q&A) say this was a very definite decision from early on though the actor can speak with a perfectly good American accent if required. Various reasons were given, but the gist was that they wanted to subvert the audiences' expectations about each character.
In an interview promoting another film, actress Alicia Vikander said when her English speaking friends heard her speaking Swedish on the phone, they later told her it almost sounded like she was speaking gibberish. When her character, Celestine is found by the others in the forest, she is speaking Swedish, but it's initially mistaken as gibberish.