I like it but it assumes I'm starting the language from scratch. How do I set my level to intermediate?

If you already know a word, just mark it as "already known". If you already know all the words it's showing you, that will cause the difficulty to ramp up very quickly as it starts skipping ahead to find words you might not know. (If you scroll down to the bottom of the page and open the "graphs" section, you can see the logic behind it.)

The app "only" includes about the 3,000 most common words, so if you're past that level, I don't know how helpful it will be to you. I can easily extend this in the future, I just need bigger corpus with more data.