I'm not sure exactly. It is currently using the browser's speech synthesis functionality. I think Chrome has some languages shipped with it while Firefox may rely solely on the OS. I will probably need to switch away from the browser to get a consistent experience.

My Firefox on Windows shows no languages available, as does Edge on Windows, while Chrome on Windows shows a handful of languages available.

Edit to say: what a fantastic and useful little tool! It is small focused, and very useful.