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Is downloading and installing the Firefox with the language the only solution?
It's the preferred solution.
If you need to switch between different interface languages,
Install the language pack for the desired languages. https://addons.mozilla.org/firefox/language-tools/ 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.
If you need to switch languages frequently, you may want to get an add-on like Simple Locale Switcher.