Langlands was once invited to lecture in France, and he chose to give his talk in French. Evidently his accent was not all that good, and the audience found it a bit painful.
Jean-Pierre Serre, one of the leading mathematicians of the 20th century, and a Frenchman, was attending the lecture that day. He interrupted to ask a question, in English.
Declining to take the hint, Langlands answered in French and carried on with his lecture.