For this achievement you'll have to discover all 23 cities in France. This includes the cities that were already in the game, not just the cities from the Vive la France! DLC.
The cities needed for this achievement are:
Calais, Dijon, Lille, Lyon, Metz, Paris, Reims, Strasbourg, Brest, Bordeaux, Bourges, Clermont-Ferrand, La Rochelle, Le Havre, Le Mans, Limoges, Marseille, Montpellier, Nantes, Nice, Rennes, Roscoff, Toulouse
These are not needed but are in France:
Civeaux, Golfech, Paluel, Saint-Alban-du-Rhône and Saint-Laurent are not needed for this achievement as those are only locations with nuclear plants and not counted as cities.