Ok since you are using a unnamed Linux distro and not Windows or macOS. I'm guessing you are using a package build of Firefox installed in your package manager and not the official Firefox tarballs from say https://www.mozilla.org/firefox/all/desktop-release/ Some Linux distros may have additional Firefox packages including language packs which are used for the language of the user interface like the menus and such. However in this case since you screenshot shows Add Dictionaries at the bottom, this shows these are Dictionaries and not Language Packs. Again your Linux distro may have included a list of dictionaries with your Firefox package install. It may be possible to uninstall any Dictionaries and language packs you do not want in the package manager though you do need at least one language pack installed for the User Interface language and at least on dictionary for the spell checker. Only a few language locales of desktop Firefox from Mozilla.org even comes with a (single) dictionary (like en-US) due to licensing reasons.