YouTube: expose which caption track is the original language (auto-dubbed videos)

YouTube now auto-dubs many videos, and each dub gets its own caption track. On an auto-dubbed Spanish video, /youtube/transcript lists availableLangs: [“ar”, “pt”, “es”]. Without lang, it returns the first listed track, which is the Arabic dub, not the original. Neither /youtube/transcript, /youtube/video nor /metadata tells us which track is the original.

Related: “Identify original language in YouTube metadata” was marked Completed (the transcript endpoint returns a default language when lang is omitted). On auto-dubbed videos that default is still a dub, because every track there is auto-generated, so it does not identify the original.

YouTube’s player response already has this: audioTrack.audioIsDefault and isAutoDubbed on the audio tracks, and kind: “asr” on caption tracks. We can’t read it ourselves, because YouTube answers “Sign in to confirm you’re not a bot” from datacenter IPs.

Could you either (a) return the original language, e.g. originalLang on /youtube/transcript and on /youtube/video, or (b) default to the original track instead of the first one when lang is omitted?

Example: https://www.youtube.com/watch?v=tXzCTWGDgXM (original Spanish, auto-dubbed to Arabic and Portuguese).