When using LibreOffice Writer in Manjaro, I notice spell checking doesn't work. No squiggly red line under misspelled words. I remember it was a problem in the past too. I've never had this problem in Linux Mint.
Here is what I did to fix it.
Spellcheck fix (USA) for LibreOffice Writer
The below 3 steps which worked for me was taken from
https://forum.manjaro.org/t/solved-spell-check-in-libreoffice/54978/9
1) sudo pacman -S hunspell hunspell-en_US hyphen-en
2) Go to
Tools > Options > Language Settings > Languages
Under Default Languages for Documents / Western, select English (USA) or any language that displays the SpellCheck icon [The A with the … underneath].
3) restart LibreOffice Writer if it's open