AFAIK, Sigil uses its own spelling dictionaries and searches for them in the following locations:
Windows:
Code:
%LOCALAPPDATA%\sigil-ebook\sigil\hunspell_dictionaries
%PROGRAMFILES%\Sigil\hunspell_dictionaries
(These special folder names can be entered in the Windows Run dialog box.)
Linux:
Code:
~/.local/share/sigil-ebook/sigil/hunspell_dictionaries/
/usr/local/share/sigil/hunspell_dictionaries/
I.e., you'll have to check these folders for French spellcheck dictionaries and replace them with your preferred dictionaries.
If your preferred dictionary is GPL licensed, KevinH might even include them in one of the next builds, if you ask him for it.
D.