On the 'General' page in Options, look at the settings under 'Language', particularly the button labelled "Set Alternative".

I assume that your issue is with the language for webpages. A language pack would normally only affect the language for the user interface, but probably also sets this language at the top of the preferred languages and you may have to reorder this list. If you install this language as a possible language in the English (en-US) version then Firefox shouldn't modify the order of the languages for webpages when you (de)select this language. You can check the order of preferred languages for websites. *https://support.mozilla.org/en-US/kb/use-firefox-another-language *https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages