"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."

H. W. Fowler, A Dictionary of Modern English Usage, first published 1926, valid still and for eternity.

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