Hm, sorry, but I can't find the plugin preferences folder. If I click at the bottom left checkbox of the "edit plugins"-menu, I find the "plugin-preferences folder", but this refers only to the epubcheck-plugin that is also installed. A .json file referring to the language tool is nowhere in the relative directory.
(This is what I meant, initially
)
P.S.: Only to clarify: I use the Sigil language tool, not the one for LibreOffice.