There's https://lingonaut.app/ , which is not open-source but at least open data. It's sort of like early-days duolingo with volunteer-driven courses. Some of the courses are surprisingly well thought out (if yet to be implemented to a usable extent), but quality will be mixed. I don't think they have an answer for proper pacing at the moment, but being a free product they're at least not incentivized to keep you repeating the same content indefinitely.

I'm getting a lot out of Anki with premade decks these days, combined with watching tons of video content.

Not a duolingo alternative, but I recently built a (WIP) web app for journaling in your target language with LLM-powered feedback and in-context word discovery.

I built it for myself as I wanted to practice writing in my target languages, while also wanting to learn new words... The idea was that hopefully I would remember the words if I could associate them to my journal for that day.

It's a little clunky, but give it a go if you're interested!

Right now it's a bring-your-own claude token model, but let me know if you're not comfortable with that.

https://lingolog.app/

If it is too slow paced for you, then you are not looking for duolingo alternative. You are looking for something entirely different. The whole Duolingo deal is fun, relaxing, low effort learning in exchange for slower progress.

And of course there are competitors, many of them. There are also many free language learning resources. But, you did not said which language you are learning and whether you are beginner or not. And most of free resources are made for specific language.

Sounds like you actually want to learn a language. You don't need a duolingo alternative, you need youtube, textbooks, podcasts and grinding vocab on Anki.

I started watching Dreaming Spanish videos and started improving like 10x faster than with duolingo. At least with comprehending other people speaking.

Specifically for Kanji I am building shodoku.app

It is very different from duolingo though. No gamification, only two types of cards (reading and writing) every card has basically all the information about that kanji available from the dictionary. Content is sourced directly and unaltered from a couple of open sourced dictionary, so no AI or content writing either.

Equally, feels like there should be a duolingo marketplace - want to learn electronics, maths, history etc through the duolingo framework - could be 'short' courses type materials, or ongoing/never ending type learning.

I've bought a few courses from various places, but I want bite size and daily learning.

I am trying prospanish (prospanish.co.uk) free lessons on youtube. I think this approach of pyramid/Lego learning is fastest and easiest way to quickly learn how to converse.