Strange user-interface.
Like in Brave (Chrome), the add-on only works on pages loaded via http(s), not on locally stored pages that you want to test under changed conditions prior upload.
But different from Brave, where just nothing happens, trying to switch the navigator-language on local files may seriously bedazzle Firefox... I saw a page continuously reloaded and languages switched without me even touching the mouse ... makes you dizzy.

Amazing concept, clean interface. Does not change the locale (according to navigator.language) on Firefox 138.0.

Fantastic!