I dug into libreoffice more in FreeBSD and found the languagetool for LibreOffice and it installs.
Extensions for LibreOffice suite
extensions.libreoffice.org
It works offline.
Just got it and tried a sentence, and it showed the first sentence wrong and the second one correct just like the samples I used.
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wrong: I could care less.
Correct: I couldn't care less.
Wrong: Me and Brandon met at Starbucks this morning
correct: Brandon and I met at Starbucks this morning.
Wrong: 1980's
correct: 1980s
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This didn't work:
Wrong: Karlee talked with I and Brandon.
Correct: Karlee talked with Brandon and me.
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If you need a thesaurus I found an extensions from OpenOffice.org.
Link
I didn't see one for US though.