I maintain the French version of LanguageTool. There are lots of grammar rules in French already, but certainly more rules can still be added. Looking at https://languagetool.org/languages/ you can see that French is currently the language with most xml rules in LanguageTool. The main problem is that I don’t have as much time as I used to for maintaining rules, so not much is happening in the French version of LanguageTool these days.
LanguageTool also detects spelling mistakes but does not propose suggestions, because suggestions with Hunspell would be too slow when checking large amount of text. We could speed up spelling suggestion using a FSA dictionary, but unfortunately, FSA dictionaries have missing features which would degrade detection of spelling errors. So far, I preferred sticking with Hunspell as finding spelling errors is more important in my opinion that proposing corrections.
Besides LanguageTool, there is also Grammalecte which is specifically created for French.