Julian Dibbell quotes H.W. Fowler’s A Dictionary of Modern English Usage on the correct pronounciation of French words while speaking English.
To say a French word in the middle of an English sentence exactly as it would be said by a Frenchman in a French sentence is a feat demanding an acrobatic mouth; the muscles have to be suddenly adjusted to a performance of a different nature, & after it as suddenly recalled to the normal state; it is a feat that should not be attempted; the greater its success as a tour de force, the greater its failure as a step in the conversational progress; for your collocutor, aware that he could not have done it himself, has his attention distracted whether he admires or is humiliated.
I think that’s what Feynman was getting at here in his discussion with Murray Gell-Mann, although, in typical Feynman fashion, not in so many words.
Richard Feynman, Gell-Mann’s chief competitor for the title of the World’s Smartest Man but a stranger to pretension, once encountered Gell-Mann in the hall outside their offices at Caltech and asked him where he had been on a recent trip; “Moon-TRAY-ALGH!” Gell-Mann responded in a French accent so thick that he sounded as if he were strangling. Feynman — who, like Gell-Mann, was born in New York City — had no idea what he was talking about. “Don’t you think,” he asked Gell-Mann, when at length he had ascertained that Gell-Mann was saying “Montreal,” “that the purpose of language is communication?”