Whenever you try to switch to a card deck other than Neo, the game will tell you that you must select a French type card deck - even if that is what you just did. It seems to happen regardless of which French deck you choose. If instead you had been playing with a card set other than Neo, the deck will automatically switch to Neo once you start a game of Clear the Dungeon.
(What a pity, considering that there is even a Fantasy deck...)