Bon Voyage!
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Vive la France
Discover all French cities (requires Vive la France ! DLC)
There are 31 in total, and this is most easily trackable in the in-game map.
Confusingly it is possible to get progress towards this achievement without the DLC, as some French cities are included in the base game - but not all the 31 that are required for the achievement.
This is easiest to keep track of in the in-game map.