Does exactly what it says, and has a great UI as well.

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.

Fantastic!

開発の勉強と英語の勉強を兼ねているのに強制的に日本語にされるサイト、本当に迷惑。
このアドオンはバッチリ役立ちました！