Add ability to download default subs/captions only

[Nytrox]

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If the purpose of this issue is a site support request please provide all kinds of example URLs support for which should be included (replace following example URLs by yours):

• Single video: https://www.youtube.com/watch?v=XvgquqdyTfA
• Single video: https://www.youtube.com/watch?v=37sR5Akt_Q4

For these videos, the auto-generated captions are for their respective language (spanish and korean). I would like an option to choose "default" for the subtitle language part so that it can download whatever the default speaking language is rather than just one language or every language.

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Description of your issue, suggested solution and other information

I would like to download subtitles in the default language of the video rather than only one language or every language. If the video is spoken in Chinese, I want to download the chinese subtitles only. And if the video is English, I want to download the english version without having to change my code.

Maybe detecting the default language of the video. When the auto-generated subtitles are of a different language, it usually says so in the video player.