After building the MVP in about a week, I spent the next week iterating on it, whilst preparing to submit my app the the App Store. It's my first time publishing on the App Store, and I learned a ton.
There are quite a few things you need to do, which I hadn't thought about. Namely:
I spend most of my time building a simple landing page, cleaning up the rough edges of the app, and planning my marketing strategy. For now I'm going to release the app as free, and aim to get people I know (both personally and via /r/LearnJapanese) to try it out and get feedback. Long term, I'd like to charge a 1 off flat rate, probably $1.50 or so.