China

I've been wondering about that myself. I tried various apps and some of them for over a year too (mainly because apps make it easier to keep at the habit and stay motivated). I find there are some repeat problems. This is based on using the Google Play store to find apps:

• Because Duolingo is so obscenely successful compared to others, most other language apps with any success converge on changing their UI to be more like Duolingo's. Two in fact that I had liked for a while, HelloChinese and LingoDeer, did this some time back (destroying a cool and unique UI design they'd previously had in the process).

• Monetization is hell (yay late stage capitalism). Those same apps I mentioned before, they also added microtransactions on top of a subscription cost. These MTX don't seem to be there for much of anything, but they are there, and put me off. Once an app has MTX, it will be motivated to put artificial barriers in place somehow, in order to drive you to MTX.

• Some apps are crash grab city and reek of copy/pasting. A good example is Ling, which boasts offering tons of different languages to learn, but the learning process is ass. No teaching of grammar in sight and it throws you right into complex combinations of words with no explanation for why they combine the way they do.

• Too many apps add "spaced repetition" through review and act like this is accomplishing something meaningful. In my experience, review means nothing if I haven't yet internalized in the first place. It just becomes a thing of me saying "yeah I have no idea, what is the answer?"

• Nobody actually seems to have a clue how to teach a language through an app and they're mostly throwing darts at a board / repeating ideas other financially successful apps tried. Because it's more about making money than learning (yay capitalism).

• You can learn some stuff sometimes, but going the rest of the way is the hard part. This is the part I'm consistently stuck on and I'm not sure apps really can do anything about it. I even tried TalkMe for a year or so, where AI is used to help you practice conversation, but there was so much of a gap in my conversational skills, it either took me painfully long to work out a response (using Google Translate or Pleco to help) or I went with some suggested response from the AI and spoke it to at least get practice with pronunciation/talking (but this kind of trivializes the point of being able to converse from your own thoughts).

I feel these kind of apps tend to consistently fail on teaching building blocks, such that you can piece things together yourself, and just generally go through concepts too fast, probably so they can say that you're learning X number of words in just a few months or whatever. Kids spend literal years going through grade levels of reading and these apps are like, okay, you did like one short lesson on this grammar rule illustrated via a few static repeating examples, you have it memorized for life now, right? Oh, you don't? That's okay, we'll just have you spaced repetition the same repeat examples for the next year. Surely it'll stick at some point.