That's strange because the spelling checker has always worked in all the versions of Libre Office that I have used. I never needed to add anything or change any settings.Perhaps these will explain.
First off - the “common response” of going into the Tools/Options/etc., etc., to “enable” the grammar checker does NOT work. It did not work in 2017 and it still does not work in 2020. (I know because I have been a avid user for many years!) I am very, very leery of doing an add-on, unless...ask.libreoffice.org
Here’s how you can enable automatic spelling and grammar check as you type in LibreOffice Writer. LibreOffice Writer supports multiple language dictionaries that are used to validate the grammar of the words as you type alongside spelling. The Writer shows red color curved line under the...www.libreofficehelp.com
It's available for pretty much everything. When in doubt, you can cheat and use pkgs.org and have a gander at which distros have the package available. It's not 100% accurate, but it's 'close enough'.To my knowledge, this program is only available to distro’s using apt (basically Debian).
Description: Carnegie Mellon University's link grammar parser
In Sleator, D. and Temperley, D. "Parsing English with a Link Grammar"
(1991), the authors defined a new formal grammatical system called a
"link grammar". A sequence of words is in the language of a link
grammar if there is a way to draw "links" between words in such a way
that the local requirements of each word are satisfied, the links do
not cross, and the words form a connected graph. The authors encoded
English grammar into such a system, and wrote this program to parse
English using this grammar.
link-grammar can be used for linguistic parsing for information
retrieval or extraction from natural language documents. It can also be
used as a grammar checker.