If you are using a package build of Thunderbird on your Linux distro then that likely explains why you have so many Dictionaries installed. Not sure if those dictionaries are installed as a group package or individual small packages in your package manager.

Just like with Firefox, I believe only a few language locales of Thunderbird tarballs that you download from Thunderbird https://www.thunderbird.net/download/ even comes with one dictionary due to licensing reasons as one normally needs to install dictionaries from addons.mozilla.org in Thunderbird like you see at https://services.addons.thunderbird.net/thunderbird/language-tools/

The same thing can occur with the Firefox package builds and with dictionaries and language packs.

Ah ok. I believe I installed this with dnf. I looked in ~/.config/thunderbird but couldn't find anything regarding the spell check. Do you happen to know where Fedora Thunderbird stores these?