I think even aside from the more outlandish ideas like that one, just having a fluent native speaker to talk to as much as you want would be incredibly valuable. Even more valuable if they are smart/educated enough to act as a language teacher. High-quality LLMs with a conversational interface capable of seamless language switching are an absolute killer app for language learning.
A use that seems scientifically possible but technically difficult would be to have an LLM help you engage in essentially immersion learning. Set up something like a pihole, but instead of cutting out ads it intercepts all the content you're consuming (webpages, text, video, images) and translates it to the language you're learning. The idea would be that you don't have to go out and find whole new sources of language to set yourself with a different language's information ecosystem, you can just press a button and convert your current information ecosystem to the language you want to learn. If something like that could be implemented it would be incredibly valuable.
Don't we have that? My browser offers to translate pages that aren't in English, youtube creates auto generated closed captions, which you can then have it translate to English (or whatever), we have text to speech models for the major languages if you want to hear it verbally (I have no idea if the youtube CC are accessible via an api, but it is certainly something google could do if they wanted to).
I'll probably get pushback on the quality of things like auto-generated subtitles, but I did the above to watch and understand a long interview I was interested in but don't possess skill in the language they were using. That was to turn the content into something I already know, but I could do the reverse and turn English content into French or whatever I'm trying to learn.
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.
A use that seems scientifically possible but technically difficult would be to have an LLM help you engage in essentially immersion learning. Set up something like a pihole, but instead of cutting out ads it intercepts all the content you're consuming (webpages, text, video, images) and translates it to the language you're learning. The idea would be that you don't have to go out and find whole new sources of language to set yourself with a different language's information ecosystem, you can just press a button and convert your current information ecosystem to the language you want to learn. If something like that could be implemented it would be incredibly valuable.