Phew! Right - got it.
The side bar was showing properties rather than styles, but now I located this settings area. Language says UK, so seems ok?
All English speaking countries incl. [UK, Australia, Canada, Ireland, South Africa, New Zealand] have pretty much all the same spelling. Only USA differs. Not in all cases, but the vast majority.
(New to the party more than three years later. And quite new to heavy work with LO… and very much liking it.)
This is simply a “+1”, with profound gratitude.
I, too, had this non-working of spellcheck on a project. Strangely, it was working on an earlier project.
I landed on this reply about “Default Paragraph Style” and its “Font” entry. Sure enough, my non-working project showed “[None]”. Setting that to my preferred language, “English (UK)”, fixed it. (And I also verified that my working project was had also been showing a real language.
Thanks!