11-character YouTube video ID — the value of the v= query parameter in a watch URL (e.g. dQw4w9WgXcQ for https://www.youtube.com/watch?v=dQw4w9WgXcQ).
languageCode
en
No
BCP-47 / YouTube language code of the transcript track to return (e.g. en, de, en-US, pt-BR). Must match a track that the video actually has. When omitted, the video’s default language track is returned.
type
-
No
Set to asr to fetch the YouTube auto-generated (speech-recognition) track. Omit to fetch the human-authored track for languageCode when one exists.