Here are a few things I'm thinking. You probably have thought about these yourself, but anyway:
- Have you ever thought about increasing the price? I find $5/month to be at the very low-end for something that a language learner may be using regularly. If I find the product useful I'd be happy to pay $25/month or more for it. Increasing the price also has the side effect the you'll get "serious" language learners to sign up, which is probably what you want.
- Have you thought about recommending appropriate texts your users based on their difficulty? I don't know anything about Chrome plugins, but I assume that you can collect data about which pages and texts users are browsing, how often they're clicking on stuff, and so on. That sounds like very valuable data that you could use to add more value and data network effects to your product.
- Have you thought about selling to organizations (schools, classes, meetup groups) instead of individuals?