It only works in English and fails for any other language, even with the prompt configured. Even if you speak the language, it won't read the text. The developer must have thought that English was enough.

I really like this extension, it's a huge boost to usability for me, especially since I find Firefox's Reader Mode really clunky and I don't enjoy using it very much. Only suggestion I have is maybe provide some voices in different accents? I'm Australian and I find the North American accents just a little off-putting.

most easy to install found in here!