I've tested this in browsers on MacOS, Android and iOs platforms. I'd greatly appreciate it if you could play around with on your favorite platforms and let me know what you discover. Comments and suggestions are also welcome.
There are peculiarities when setting voice and language. The available voices differ depending on the platform. If the voice drop menu is disabled, it's because the call to get those voices fails on your browser/OS.
The language menu is static but not all languages are supported. In many cases, setting the language will also set the voice.
The Voice Synthesis engines will translate abbreviated units, for example: "5km" will be spoken as "five kilometers". Dates written numerically, like 1/3/2020, will be spoken out with month names and ordinals but can refer to different dates, depending on the voice and language settings.
In some browsers, the app will work with non-latin alphabets! Try these:
text: سلمرحبا بالعالمم text: 你好世界 voice: maged voice: ting-ting lang: ar lang: zh
Browser Compatibility
https://developer.mozilla.org/en-US/docs/Web/API/Web_Speech_API