Thank you both for your speedy replies; I am not accustomed to such good service.
In Windows and Linux I disabled LightProof and went to Tools > Options > Language Settings > Writing Aids > EDIT… to enable LanguageTool for English. LanguageTool is functioning in both operating systems now.
I did not see the notice “Please see our checklist if you experience problems.” because, frankly, I cannot read a font that small. I would suggest making that line larger. Better yet, at the top of https://languagetool.org/ after “Development” you could add “Support” and link to the issues page there. That would have saved me from bothering you on the forum. Again, many thanks.
P.S. I tested the phrase, “Those two is very nice.”, but this phrase is not marked up. I hope someone with a knowledge of rule writing will look into this. I also hope that sometime soon LanguageTool will work with LibreOffice documents containing revisions; this seems like a pretty big deal. (It appears that no one at LO cares.)