Americans frequently mispronounce French words and this is sometimes due to misconceptions about French, which is not a phonetic language.
1) Americans often think that final consonants or consonant sounds are usually not pronounced, even when a consonant is followed by an e, in which case the consonant is always pronounced.
Example: Coup de Grace becomes coup de Gra (no s sound)
Paradoxically, final consonants of many French words which end in consonants (without a mute e) are pronounced:
Aix (like in Aix les Bains) mispronunced AY
2) Faux pronunciation of French words with final “n” sound being pronounced as if they were nh, like (correctly)in the word Chanson
Example: Carcasonh for Carcasonne (last syllable is pronounced like “sun”, Bayonne.
Cawnh for Cannes (should rhyme with American can). Would be correct if the word was Caen(often pronounced Cayenne, like the pepper!)
3) Mispronunciation of words ending in eux as if they were the French aux (like Chevaux)(all sound O:).
Example: Montreux becomes Mont-TRO, Bayeux becomes Bay or Buy “O”
Acceptable (American) Anglicizations:
Mispronunciations which are now so common as to be considered normative:
Fleur de Lys–pronounced Fleur de Lee(should be Fleur de Lees (like in the word lease)
Chaise Lounge–the original Chaise Longue has now become an affectation in American English
Many of these are uttered by "Experts" like Rick Steves, who actually has phrase books on how to speak French--what a joke!
British speakers of English have their own style which is culturally normative, as for example the almost invariant accenting of French words on the penult (this goes back to original Anglo-Saxon words which were so accented) Examples are:
Accent on penult: Cafe, Calais,
Other rather stupid corruoptions such as Restauranteur, Vinegar-ette, Cache pronounced Cashay, instead of Caash (common with military people).
Going a little further afield with respect to mispronunciations of American words, here are some of my favorite (pet-peeves)
Forte, pronounced Fortay:
“This word is often mispronounced "FOR-tay" because it is confused with the Italian word forte (pronounced "FOR-tay"). The words are spelled the same but have different pronunciations and meanings. If you play a musical instrument, you will probably recognize the Italian word as a term meaning "loud." When referring to ability, the correct pronunciation is "fort," but in music, it is always "FOR-tay” (like Pianoforte)