To put this into context. The French Police can fine you on the spot and expect to take payment too. If you don't pay then they can confiscate the car. There's many a tale of Ferraris etc… being confiscated over the Le Mans weekend.
Now this doesn't mean that they will confiscate your car but the foreign plates make you stick out.
We were stopped last year at a toll booth and given a lecture, no fines even though the owner of the van didn't have all their paperwork. I did so was charged with driving the rest of the journey. We were in a VW T4 van which was in dire need of a full service. That didn't stop the copper stating that we 'must' have been speeding.
When I go to France I carry a spare bulb for all my external lights.