How to teach my friends from MIT good English language skills? Seems that they are very good at math and physics, but their language skills suck. They can neither write nor speak properly.

In a certain way you are right. But at the same time some of them are gifted people with an IQ close to 200 units. So average people like us will have difficulties understanding their needs. I would assume that a conversation club like https://promova.com/conversation-club would awaken their creative nature and make their studies even more ingenious.