alert(JSON.stringify(speechSynthesis.getVoices()))
console.log(speechSynthesis.getVoices()); // много лучше
console.log(
speechSynthesis.getVoices()
.map(voice => `${voice.name} (${voice.lang})`)
.join("\n")
);
/*
Alex (en-US)
Alice (it-IT)
Alva (sv-SE)
Amelie (fr-CA)
Anna (de-DE)
...
*/