(spoken language, not a programming language)
Vocabulary seems to be solved with spaced repetition (anki, super-memo), motivation through gameification (duolingo). But this all feels rather primitive still.
What I miss:
a chat bot adjusted to your level
moving between tools or at least proper spaced repetition in duolingo
proven to be good, live online courses that replace going to a class (this might exist, haven't found it).
systematically matching tandem partners (maybe you can be the native speaker for someone and have a native speaker in the desired language chat with you in return)
good bodies of vokab with audio, sorted most to least frequent, importable to my spaced repetition software of choice
anything else existing that I missed?
Back in the day I learned Japanese to nativish fluency using the Core Japanese vocabulary decks. Basically 10000 words all with example sentences and human audio recordings.
After seeing that many sentences, new grammar has only so many places to hide. It's also a lot more fun to practice speaking a language when you have the vocab to talk about basically anything.
I built a similarly formatted Anki deck using tatoeba.org, some web scraping and google translate.
This type of structured rote learning works for me, but it's not "fun". I write out every word and all is conjugations on paper, and have notebooks full of just words. Unfortunately none of the "fun" methods work for me, but it is motivating that in 198 days I will have 4000 words of danish.