When I commuted to work a lot, I'd use Termux (a terminal emulator for android) to write code, and practice vocab on the way to work.

I used this program to my great satisfaction. It has a simple interface and provides exactly the experience I wanted from a spaced repetition app.

I wrote it in Hy, which proved to be a really nice way to write Python, but requires a "pip install hy3" before you can use it. Not an issue for most users.