From A Dictionary of Modern English Usage by H.W. Fowler, 1926
FRENCH WORDS. Display of superior knowledge is as great a vulgarity as display of superior wealth—greater, indeed, inasmuch as knowledge should tend more definitely than wealth towards discretion & good manners. That is the guiding principle alike in the using & in the pronouncing of French words in English writing & talk. To use French words that your reader or hearer does not know or does not fully understand, to pronounce them as if you were one of the select few to whom French is second nature when he is not of those few (& it is ten thousand to one that neither you nor he will be so), is inconsiderate & rude…. Every writer who suspects himself of the instinct should remember that acquisitiveness & indiscriminate display are pleasing to contemplate only in birds & savages & children.