Coup d'état. This phrase is often used to describe the overthrow of a government by force. Think of all those rapid changes of government in many Latin American and African countries. Literally “cutting" or “blow of state." Pronounced like "coo daytah."
Coup de grâce. This is used to describe any kind of mercy killing in a variety of literal and metamorphic senses. English speakers tend to mispronounce this one a fair bit: as with the above, the "p" in "coup" is left silent, so pronounce about it "cou duh grahs."
Crème de la crème. This is about the same of the “cream of the crop" idiom in English, literally translating to “cream of the cream."
Esprit de corps. This phrase is used to express camaraderie amongst people, usually in the context of the military. Literally “spirit of the body." The end bit on "corps" is left silent, so say this as "espreet deh cohr."
Fait accompli. This little phrase is used to describe something that has already happened without any chance to reverse it. This literally translates to “accomplished fact." The "t" of "fait" carries over because of the vowel beginning "accompli", so pronounce this more like one word: "fehtakohmplee."
Laissez-faire. This phrase has been used since Colonial times to refer to a policy of “let be" when it comes to economics, allowing unhindered free trade and other practices with little or no government involvement. As with previous examples, the "ez" is not pronounced, leaving it as "lahzay-fayr."
Tête-à-tête. Literally “head to head." Slightly less painfully, this idiomatically is used to refer to close, intimate conversations.
Tour de force. This is used to refer to exactly what it sounds like: a tour of force, a feat of strength.
Vis-à-vis. Translated somewhat roughly as “seeing to seeing", this phrase is used to mean “face to face." Because of the vowel sandwiched between two consonants, try to pronounce this somewhat slurred, more like one word: "veezavee."