The law will be repealed once the politicians have come around to their senses.
It is impossible to force a people to comply with anything, all this will do is ensure that more people will want to wear the niqab in order to be persecuted for their Gods sake.

Of course eventually people will start to blow themselves up in order to 'force' the French to back down, thats the point where the French won't be able to repeal the law because of the loss of face (no pun intended)