Aegisub finds no spell errors? How to add non english dictionary?
Kate and other Linux software missing a dictionary, how to install?

I have found that spell check dictionary can be installed by adding software package hunspell*

For example if i want czech, i do:
sudo apt search hunspell|grep cz
sudo apt search hunspell|grep cs

This works for Debian/Ubuntu likely.

On Redhat based, Gentoo, Fedora, maybe use yum instead of sudo apt.

For Arch/Manjaro, maybe use pacman -Ss instead.