Secret
(une) langue secrète
confiance
dimanche
Luxe
French people call it a motel (masc.)
échine.
Argot is a French, Spanish, and Catalan word meaning "slang." So I suppose the synonym would be "slang." It is also a secret language, so secret language could be another synonym for it.
Monegasques call Christmas "Noel". Note. The official language in Monaco is French.
The official language of the sport of fencing is French, and many of its roots are in France, so yes, you could call it French.
Un lieu de rencontres
They Call it their Secret Language. And all it is is adding an extra B in the middle of each word. :)
smile = sourire (noun and verb)