The language read-only property of the TextTrack interface returns the language of the text track.
A string containing a language identifier. This will be a valid BCP 47 language tag, for example "en-US" for United States English or "pt-BR" for Brazilian Portuguese.
In the following example the value of language is printed to the console.
let video = document.querySelector("video");
let track = video.addTextTrack("captions", "Captions", "en-US");
track.mode = "showing";
console.log(track.language);