Add language pack and spelling checker to firefox

It's the preferred solution.
If you need to switch between different interface languages,

1. Install the language pack for the desired languages.
   • https://addons.mozilla.org/firefox/language-tools/
2. Type about:config into the address bar and press Enter.
3. To bypass the warning, press the big button labeled "I'll be careful, I promise!".
4. In the search box, paste intl.locale.matchOS
5. In the search results, if intl.locale.matchOS is set to true, double-click it to set its value to false.
6. In the search box, paste general.useragent.locale
7. In the search results, double-click general.useragent.locale and set its value to the desired language.
8. Restart Firefox for the change to take effect.

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