Thanks so much for testing Japanese! I'll review the starter deck and what goes into it. At the moment it's a bit random, and the nonsensical LibriVox intro spills in. I'll also try to filter out the function words. I have no way to judge that myself, my Japanese is zero.

Glad the mobile interface works. There's an iPhone app too, basically the web app but with native audio support. It can play in the background. Though it's not ready for release yet.

The built-in flashcards use FSRS-5, and I believe it's O(1) so it should scale but I haven't actually done any performance testing yet, to be honest.

The infinite loop is a bug, I'll dig into that too. And I'm not sure if you noticed, but I also put up an FSI Japanese course here: https://lingochunk.com/c/fsi-japanese. Again, I can't judge the quality myself, so I'd really welcome any feedback.

And thank you for the kind review!