Currently there seems to be a mismatch between https://dev.w3.org/html5/html-sourcing-inband-tracks/#mpeg4 (which recommends that the language property of the media tracks should be the 'Content of the language field in the MediaHeaderBox' which is 3-letter ISO 639-2 language id) and HTML5 spec https://html.spec.whatwg.org/multipage/embedded-content.html#dom-audiotrack-language (which specifies that A/V track language should be a BCP47 language tag, which looks like e.g. "en-US"). The BCP47 language tag might be present in ELNG box in some .mp4 files (although it's not very common yet). We need to sort it out and ensure that user-visible JS-level properties always return either proper BCP47 language tags or empty strings.
See also discussion at https://codereview.chromium.org/1735003004/ for more details.