LLMs cannot "teach you a language". They make for cool demos to show off. They can perhaps be a building block of a proper language learning experience.
Then again, the only languages I actually learned - besides my mother tongue - to the point of being able to do things were English and Latin and both were very much acquired offline. I have plenty of experience with language learning apps and I'm not convinced tech is the solution or even part of the solution.
Could I ask ChatGPT to just "teach me spanish"? Surely not. But if you've got even a slight idea of what to do (learn present tense and vocab, then progressive, future and past, then some conditional, hypothetical etc...), it can be an absolutely incredible tutor.
I started using it when i was already at a pretty high level, but I'm quite certain that it would have been excellent from the very beginning. It translates, gives varied examples, explains syntax, compares verb tenses and conjugation and more.
Using it and being surrounded by people writing and/or speaking the language is probably the right way to learn a language. That is how I learned Polish which is really difficult. I joined a community, and 2 years later, my Polish was quite good! YMMV.
After 4 weeks I also learned Spanish enough to maintain casual conversations just from trying to talk to someone online who did not speak English. I am rusty now, however, because I do not speak it with anyone, nor do I see or hear Spanish anywhere. Spanish is way easier, IMO, in comparison to Polish.
Oh, right. I think whatever works best for your personality but in general doing some exercises and/or interacting with people has been working for a couple .. ten thousands years at least. Hard to go wrong. I never heard someone make the believable claim that they interacted with too many people and it hampered their language learning.
My take is that basically anything can be made to work if you are properly motivated. Tech is - at best - a secondary concern.
I think the best way to learn a language is offline through actual human interaction.
I've used Duolingo in the past (and other apps) and quickly lost interest, it's a fun app, but I feel like you don't learn from it. If I had to learn a new language today, I'm confident I could make good progress with GPT or Gemini, but tailoring it to how I learn.
Then again, the only languages I actually learned - besides my mother tongue - to the point of being able to do things were English and Latin and both were very much acquired offline. I have plenty of experience with language learning apps and I'm not convinced tech is the solution or even part of the solution.