Sure, but most YouTube streams aren’t delivered as a single webm or mp4 file, and the language & subtitle selection you’re referring to aren’t implemented by the browser (but instead by a JavaScript application downloaded from Google). So it’s not what OP asked about.
YouTube supports multiple audio tracks and multiple language subtitles. Same for Netflix and pretty much all major streaming platforms.
Sure, but most YouTube streams aren’t delivered as a single webm or mp4 file, and the language & subtitle selection you’re referring to aren’t implemented by the browser (but instead by a JavaScript application downloaded from Google). So it’s not what OP asked about.