If you've updated to Mozilla #Firefox 117, there's now an un-enabled translation option built in to the browser.
It works locally on your computer so you're not leaking data to any giant corporations as may be expected with the other major browsers.
It's beta so caveat emptor.
Available languages:
Bulgarian
Dutch
French
German
Italian
Polish
Portuguese
Spanish
Go to about:config
Set browser.translations.enable
to TRUE
Go to Settings
Under Translations
Install offline language packs