I kind of gave up on Japanese years ago, but I've been trying to learn Korean for a couple months now. I started with Duolingo (great for basics like the alphabet, and in general for reminding you to practice everyday), then added YuSpeak (similar to Duolingo but way better paced/structured and with some useful systems to keep track of your weak points and review words and concepts effectively. Their Japanese course has even more features).
Although the apps are a great way to start, like other commenters said, they really promote memorizing over understanding/using the language, especially Duolingo. So the greatest addition for me so far was actually ChatGPT (GPT 4). Now I can learn a new concept on YuSpeak or Duo and then immediately go ask ChatGPT to go over it together and make up some exercises for me to do. It really works surprisingly well, and in just the few days since I've started using it, it feels like my ability to write and read the language has improved exponentially compared to when I was only using the apps. I also fare much better in the apps themselves.
Of course ChatGPT is not 100% accurate, as we all know, but it opens up so many learning avenues that it doesn't really matter, especially if you're a beginner to intermediate student. Obviously it also can't completely replace an actual teacher (on the other hand, it's available 24/7, unlike an actual teacher, and it's much cheaper too) and it won't help with listening or speaking, but using it in conjunction with apps and a YouTube video here and there really creates a pretty effective and proactive learning environment.
TL;DR: Check out YuSpeak and consider using ChatGPT 4.
I can't speak about Flutter or React Native, but what I can say is DON'T use Xamarin Forms/MAUI. As a native Android developer I had to start using Xamarin after changing jobs and it's been one of the biggest regrets of my career, honestly. Literally nothing works like you would expect it to. I understand the idea of writing the same code twice is intimidating, but trust me, nothing beats native development. Nothing. I can say with 99.9% certainty, you will regret not going with native if (or when) your app requires any vaguely complex feature to be implemented into it. Swift and Kotlin are similar enough that you can literally write the same app natively for both platforms faster than it would take you to write them in any cross platform framework (or at least Xamarin/MAUI), unless you're making an extremely simple app with no customizations whatsoever.