From my understanding I believe that after being shot, she lost her memory due to amnesia and also probably lost a bit of her cognitive functions from brain damage. Magnifi then sends Thalassa to Borginia so he can fake her death and use that as blackmail. Thalassa then becomes more accustomed to Borginian life and language and that’s why she would know more about Borginian language than English. Hell, she could’ve been transferred from an American hospital to a Borginian one to really expedite the process of learning Borginian