Works for me, needed it to stop websites from translating themselves and it did just that, so thanks.

Does not work

Impossible to add a new language. There is no save button in the settings.

Doesn't do anything. I.e. navigator.language does not change.

seems it does not work front end javascript. tried with tempus dominus library but no luck so far.

Does the job. Go to Firefox preferences then Add-ons to add/remove languages.

Excellent. Does what it says and does it simply, but well. Highly recommended.