Are you a total novice or do you have some basics? Because Duolingo is a decent introduction. That said, I’m quite the fan of Clozemaster. I’ve only been using it via the webpage for 20 days now and alongside Duolingo, but I like the approach for what is essentially five minutes a day. Obviously neither are free nor open source. For free and open source, I’ve been intending to take a look at Ankidroid, but haven’t gotten around to it yet.

Maybe Kanji dojo if you wanna learn Japanese, And trivia you to quiz yourself

Anki

That’s the basis of Bloom’s taxonomy: knowledge or memorization

Spaced repetition apps, like Anki and Duolingo help a lot with the base, the rest needs guided learning with regular activities.

I have tried a few. Duolingo seemed to be good for making it fun to get interest but a lot of it was semi-random vocabulary. Try the Mango app for more practical conversation practice. It has flash cards that remind me of Anki that are nice for reinforcing memorization. It is free but I think has a premium option now.

Also, Tandem for practicing conversation with real people that speak the language via text, voice message, call, or video call.

There’s really nothing worth spending your time with for verbal language learning when it comes to free and open source software. In fact, most software isn’t worth your time.

iTalki can be good for finding you a tutor and having video chats.

Pimsleur is good for solo instruction and practice.

YouTube is good for tips from other language learners and finding immersive content (i.e. content from native speackers in another language).

None of this is libre software.

I recently recommended Rosetta Stone to a new friend who just moved to Denmark from South Africa with his family, only to find that they had switched to a subscription model, and also scrapped their Danish course completely 😓

Was getting ready to raise the black flag and search the seven seas for the old Danish Level 1 course, when I discovered that it was available at the library, and he just had to put in a reservation and it would be sent to our local branch for pickup.

He has it now, and he’s super happy with it (as a supplement to Danish classes).

If you don’t know Rosetta Stone, its main feature is that there are zero instructions; everything is set up so you have to intuit what is going on, the idea being that you’re learning the new language “the same way you learned your first language”, as opposed to having to use your main language as a translation layer.

I did a little bit of their Japanese and Spanish Level 1 course some ~20 years ago, and I still remember everything I learned, even though I never used either language for anything since.

Anyway, my main point is that you might be able to get access to good commercial courses from your library, maybe check it out 😊👍

Busuu is similar to Duolingo , but with more focus on grammar. Have to subscribe to get the most of it. It has quite a small language choice.