Hi and welcome to the forum.

Dogs here don't seem to wear tags. Up until a couple years ago, France used tattoos to id dogs - either in the ear or, for a valuable dog who might be subject to being dog-napped, on the inner back leg. (Apparently some dog-napped dogs had their ears cut to eliminate the tattoo.)

What they sometimes do now is to tattoo the letter "P" where a tattoo would normally go - which indicates to anyone finding the dog that he has a "puce." (Yes, that's what it's called here.)

Check with a vet when you get to France. There is a form to fill out to transfer the chip/puce i.d. to your new address in France.
Cheers,
Bev