Hello, Pascal, welcome to the forum.
I agree with Andrew, Fritillaria meleagris is very beautiful, easy to grow and very dainty and elegant to associate with the Galanthus although it may flower later than most of them. If you want something a little bigger to look good against the helleborus foliage, and flowering later again in the year, then Fritillaria pyrenaica would be an excellent choice. Depending on where you are in France, the dark flowered frits you mention might also be happy to grow outside. F.f meleagris and pyrenaica are the easiest to grow, I would suggest to begin with those. They are also among the prettiest!