I agree, you definitely want to keep some sort of free tier. There are a lot of apps out there that claim to do this, or something similar. After trying a bunch of them I found one or two that I liked, but for the most part they weren't very good. I would not have wanted to pay to find that out.

I think you wouldn't want to offer less that one full alphabet, so people can get a good idea of what it will be like to learn using your app. If it's just a few characters that they can practice with, then they won't spend enough time with your app to make them feel like they should purchase it over trying another one out.