Thanks Murasame & Levantine
It only looks like it works with the french language but that's not true:
1) the cutscene modifications (hidden scenes, recruiting Beatrix, etc...) are not present if you choose the french language,
2) some bosses will act very strangely in battle and it may even crash the game (early game, there's a fight against Steiner who uses a death strike way too often, or the Sealion fight that is unbalanced...),
3) and of course, the names and descriptions of the different spells and items will not fit the change.
So yeah, I would still recommend you to switch to english at that point. Also, playing the game in english makes you realize how the french translation was good sometimes :p