According to the YouTube API documentation, one should be able to determine the language spoken in a YouTube video using the list endpoint. In particular, the language code is in the response: response['items'][0]['snippet']['defaultAudioLanguage'].
This works in most cases. However, some videos, such as this one, do not have defaultAudioLanguage in the snippet. Why does this happen and how can I determine the language spoken in the video in such a case?