Hi
The - probably - most advanced and up-to-date French open-source dictionary is packaged by Grammalecte. In fact, it's a compound of 4 different French dictionaries within one package. The user can select the "classic" if he is a native French speaker, the "modern" if he is a foreigner, etc. It all boils down to some preferences in alternative writing.
I can select this option with Sigil (where I manually inserted the .dic and .aff files) but I cannot see in the Calibre editor if I could do it. (see screenshot)
http://www.dicollecte.org/grammalecte/telecharger.php Look for "Dictionnaires"
(see the .oxt file)