The point is to achieve immersion learning. Changing the language of your subtitles on some of the content you watch (YouTube + webpages isn't everything the average person reads) isn't immersion learning, you're often still receiving the information in your native language which will impede learning. As well, because the overwhelming majority of language you read will still be in your native language you're switching back and forth all the time, which also impedes learning. There's a reason that immersion learning specifically is so effective, and one thing AI could achieve is making it actually feasible to achieve without having to move countries or change all of your information sources.