The following are some of the best known French sayings - i.e. small comments, quotes and phrases in the French language that are commonly used outside France...
à la carte - a menu offering free choice of items in a restaurant, each priced separately
à la mode - fashionable ("your dress is very a la mode")
aide-memoire - a way to help you remember something
apres-ski - night-time entertainments after a day skiing ("did you enjoy the apres-ski?")
au contraire - "quite the opposite" ("you like cats, don't you", "au contraire")
au naturel - naked
au pair - foreign person living in your home to help with looking after the children
avant-garde - very modern ("the new museum was very avant-garde")
cafe au lait - coffee with milk
carte blanche - a free hand to do something ("you have carte blanche to landscape the garden as you think best")
c'est la vie - that's life!
cordon bleu - high quality cooking, usually French style
cul-de-sac - road or street with no exit ("I live in a cul-de-sac so it is safe for the children to play outdoors")
deja vu - feeling that you have already experienced something before, as in a dream
en passant - usually used in refernce to a chess move, means 'in passing'
en route - on our way ("we are en route and will arrive at 4 pm)
faux pas - an embarrassing moment or event (a 'false step')
hors d'oeuvres - snacks enjoyed before a meal (literally 'outside the main works')
mardi gras - type of carnival
nouveau riche - someone with lots of money that has not been inherited
pied-a-terre - a small place (apartment) in the city to stay, often during the working week, with a main home elsewhere
RSVP - please answer this invitation...
trompe l'oeil - a painting that represents something real and is intended to mislead the viewer into thinking it is real (literally, the phrase means 'cheat the eye')
These are only the most common French sayings - there are many others you might come across, particularly in restaurants (table d'hote, prix fixe etc) and some that only seem to exist outside France - I have never once heard someone in France say 'sacré bleu' for example.
See also French quotes - quotes about France itself (rather than phrases used by French people...)