If someone said that you were mendacious, they are calling you a lier. Mendacious means: Lying; untruthful.
die
not go
I would say something like um.. "I'm really sorry but something really important came up" plus you can try to rearrange something do whatever you are doing now go to that for a while then go to the...
Good = Bien Good, you? = Bien, tu ? Good, and you? = Bien, et toi ?