When Sun and Jin are leaving to go to Locke's camp and Juliet stops them, she says Jin's name to get his attention and he stops and faces her. The camera cuts to Sun's face and then back at Jin, who is shown once again walking and stopping to face Juliet.
Jin previously stated that Sun and Sawyer have been helping him speak/understand English. Yet, he reacts when Juliet says, "Your wife had an affair." Although, plausible that Sawyer taught him the word "Affair" (we know Sun wouldn't teach him); it's highly doubtful he learned the word and its meaning with his very basic English understanding.