I don't know how strict they are in France.
But as a preventive warning to anyone who wants to travel in Spain with his pet, if they catch you here with your dog unleashed / secured to a specific seat or in a cage, you're toasted: 100 € to 500 € fine.
They have no mercy. There is not a "Awwwnn he is so cute! OK, please be careful and next time secure him to the back seat or put him in a cage.".
It's more this style: "Good morning! Dog unleashed to a seat buckle and on the passenger's lap: 500 €. You're not Spanish, therefore you pay now with a credit card or cash. And you need to secure your pet right now. And if you don't pay and / or don't have any cage or specific harness for the pet to travel in this car, you have to leave the dog here or call someone to pick him up in a proper way. Until you do so, you can't continue your journey and your car will be immobilized".
