Hopefully never. Hopefully too, IRL somebody in the EU will kick some balls on French government to get rid of that blatantly useless and stupid sticker. Signage in vehicles in a common regulatory zone like Europe is should not be dictated by individual actors (country/state/whatever governments). In fact, there exist treaties (signed by France among so many other countries) specifically to avoid that.
If at least would be a sticker with a meaning an a practical purpose (like the green circle ones to sort vehicles allowed to drive in some places) it would have sense.