It’s the preferred solution.
If you need to switch between different interface languages,
- Install the language pack for the desired languages.
- Type about:config into the address bar and press Enter.
- To bypass the warning, press the big button labeled “I’ll be careful, I promise!”.
- In the search box, paste intl.locale.matchOS
- In the search results, if intl.locale.matchOS is set to true, double-click it to set its value to false.
- In the search box, paste general.useragent.locale
- In the search results, double-click general.useragent.locale and set its value to the desired language.
- Restart Firefox for the change to take effect.