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.