Thanks to some googling I learned that ghostwriter uses hunspell. So I checked that hunspell-fr was installed. Doing so, I found that packages from aspell and myspell where installed to. French dictionary was missing for both of those spelling engines.
As it seems that hunspell is the more modern and unicode compliant (according to whot I found with more googling), I decided to uninstall the aspell and myspell packages.
So to sum-up :
- check that the hunspell-xx package is installed (xx is the code for your language)
- uninstall packages for unnecessary spelling engines
- set your desired language in ghostwriter