The language read-only property of the TextTrack interface returns the language of the text track.

This uses the same values as the HTML lang attribute. These values are documented in RFC 5646: Tags for Identifying Languages (also known as BCP 47).

A string containing a language identifier. 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);