Language Packs are for the language of the Firefox User Interface or UI. It is a option to have one

Language Packs are for the language of the Firefox User Interface or UI. It is a option to have one Firefox install and be able to change the UI to one or more other languages.

The inline Spell Checker in Firefox uses Dictionaries. https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker#w_adding-dictionaries

How to use Language Packs in case you still want to use them. https://support.mozilla.org/en-US/kb/use-firefox-another-language