For spell checking, some languages use FSA dictionaries and others use Hunspell. French uses the Hunspell dictionary from the www.dicollecte.org project. Unfortunately, spelling suggestion with Hunspell were deemed too slow to enable them. I did not want to switch to FSA, because the quality of spell checking with Hunspell was better than with FSA. The quality of spell checking and speed of LanguageTool matter more (to me at least) than suggestions. I don’t remember now what were the limitations with FSA. I think it may be Hunspell ICONV directive which was not supported, among perhaps other things.
Now thinking about it. I wonder whether it would be possible to propose spelling suggestions lazily, when the user requests them for a single word, instead of finding suggestions for all typos as LanguageTool currently does now. It would be much faster. It would not be applicable to the command line version of LanguageTool, but lazy suggestions would make sense in theory for the LibreOffice plugin or for the online web demo. I don’t how doable this is in practice.