[KevinH]

In Preferences for Spellcheck dictionaries you have selected the French dictionary (not French - France) but in the OPF dc-language metadata you have put fr-FR (which means the french language specific to France)

So the easiest solution is to change the OPF dc:language tag to be just fr so you can use the dictionary you already have installed. You can do that in CodeView by hand editing the OPF, or use Sigil's Metadata Editor so no hand opf editing is required.

Or go find a specific fr-FR dictionary from hunspell and install it locally.