The Language Packs is for to change the language of the UI or user Interface of Firefox as the inline spellchecker uses '''Dictionaries'''. On top of https://addons.mozilla.org/firefox/language-tools/ <blockquote>Installing a dictionary add-on will add a new language option to your Firefox spell-checker, which checks your spelling when you fill out Web forms. Language pack add-ons change the language of the entire application.</blockquote> As you can see, two different things. https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack Also the en-US Firefox from www.mozilla.org/firefox/all comes with a dictionary. However most locals do not come with a dictionary due to license reasons.