Duolingo is terrible†, but proper gamification combined w/ LLMs for real conversations could be an incredible learning tool. (I might build this if no one else does.)
†It can be useful for going from absolute 0 to epsilon, just to kind of get familiar with the language, but if you're using it more than like 2 weeks, you're seriously wasting your time (vs. reading material in the target language, watching TV in target language, trying to talk w/ people in target language). Anki, too, can be a trap that feels like learning but isn't, really, in my experience.
There's a newer app I use called Natulang, developed by a Ukranian software dev to solve this problem for themself, which is entirely speaking focused w/ AI support and aims to get a person to a B2 level over 360 lessons w/ about 15 minutes each. I'd round up to 30 minutes each for actual time commitment due to the extra SRS sessions tacked on.
I'm 50 lessons in Spanish now and I definitely believe the claim. Recently was on a date w/ someone who knew about as much English as I know Spanish and only grabbed Google translate about a half dozen times.
It doesn't have much in the way of gamification... to me the fact that it seems very evidently effective is enough motivation to do a daily lesson.
Actual LLM powered free-form conversationalist assistants are better once someone has a solid base understanding, probably at least a 2000 word vocab. What you'd really want is a LLM powered instructor that develops and adjusts a lesson plan based on progress.
Playing briefly, looks pretty good! -- though I wonder if there's a way to move away from using a source language (or maybe it does this in later lessons?). You really want to try to get your head 100% inside the target language as quickly as possible, and not be translating back and forth.
You can do this with the "free dialog" option from the beginning. The only issue with this is you do have to reference the actual lesson material to that point, so it's more of a review piece.
That said, my impression is getting to functional in a language quickly requires referencing a source language that is fully understandable by the user to build vocab and comprehension - ie. explaining a new concept in the target language using the target language for a B1 student is going to be inefficient and not expressive enough. Otherwise you're fortifying what you already know vs. actually building more knowledge. Things like comprehensible input are great but seemingly more indirect and less efficient.
If you have an option to get from zero to B2 fairly quickly, you are functional enough in the target language to use a myriad of options to fluency, including doing nothing other than conversing with others.
You can get quite far with consistent long term approach with stuff like Duolingo. The problem is, its just one or very few... vectors or dimensions in which you progress, specifically aligned with how the material is done. I have a friend, he is doing DL for French for maybe 2 years, every day. He can talk some stuff pretty well, freezes on some other situations. Passive understanding works quite well for him too.
Real use of language has many dimensions, changing also ie the ways you think in that language for example.
Nothing beats real use where you have to express yourself and not skip to other languages as a shortcut, no way around this.
I've tried learning apps with LLMs and part of the issue is that you can't have much of a conversation early on. A conversation of "how many cats do you have?" "I have two cats" "what color are your cats", etc., isn't much different than the non-AI lessons. At the point where it would be really useful, the other options you mentioned are much better choices.
I think having a world (3d maybe, or maybe just 2d) you could talk about in a really simple way might be useful here. Imagine something like "el gato quiere la pelota roja" and you have to carry the red ball to the cat to pass to the next lesson, and there's a cat, and a dog, and capibara and various shapes; something like that...
There's probably the opportunity to have simple stories and personalities come into play too, early on, to add interest. Think about e.g. the Frog and Toad books for children learning to read.
I teach languages and teaching people how to functionally craft things with a language works much better in the medium to long term. By the time you get some basics down, you can actually have a conversation beyond "comment ca va, comment t'appelle tu?" because you know how to use the language, not just parrot phrases.
> but proper gamification combined w/ LLMs for real conversations could be an incredible learning tool.
I don't necessarily disagree but I do believe it will require some really smart design ideas. I am pessimistic that a big name company will come up with them
†It can be useful for going from absolute 0 to epsilon, just to kind of get familiar with the language, but if you're using it more than like 2 weeks, you're seriously wasting your time (vs. reading material in the target language, watching TV in target language, trying to talk w/ people in target language). Anki, too, can be a trap that feels like learning but isn't, really, in my experience.