This is what one of Edmond Dantes alter egos did in the Count of Monte Cristo. “Lord Wilmore” was an eccentric Englishman who understood French perfectly well, but refused to speak it:

… Lord Wilmore appeared….His first remark on entering was, “You know, sir, I do not speak French?”

“I know you do not like to converse in our language,” replied the envoy.

“But you may use it,” replied Lord Wilmore; “I understand it.”