Look at the app ISSEN. The "best" would be to somehow have a hypothetical LLM that actually works. For now it's only at the beginner level. We can use raw input and spaced repetition to guage progress instead of tests.

I've learned 2 languages to fluency, and the only thing that ever works for me is immersion with comprehensible input, and conversation. I've been generally disillusioned with language learning apps that aren't "language exchange / penpal". And I've tried all of them. I don't think language learning is easily "gamified".