SpeechSynthesisVoice.lang

The lang read-only property of the SpeechSynthesisVoice interface returns a BCP 47 language tag indicating the language of the voice.

var myLang = speechSynthesisVoiceInstance.lang;

A DOMString representing the language of the device.

for(i = 0; i < voices.length ; i++) {
  var option = document.createElement('option');
  option.textContent = voices[i].name + ' (' + voices[i].lang + ')';

  if(voices[i].default) {
    option.textContent += ' -- DEFAULT';
  }

  option.setAttribute('data-lang', voices[i].lang);
  option.setAttribute('data-name', voices[i].name);
  voiceSelect.appendChild(option);
}

• Web Speech API