I read your post on Medium
I like this idea for sure. I think I'd use this tool occasionally.
Here's my use-case:
I am Turkish. I speak and write English fluently and have no issues whereas my Spanish is not as good as my English.
I started learning Spanish years ago in University and always continued practicing. I am fluent in speaking and reading, but I always have issues not knowing some words.
Whenever I meet a Spanish speaking person, I practice with him/her. In web, I bump into Spanish articles and read them along.
I think I am a good fit for the free model (just use 5 words only per day). However, I want to tell you that this is a nice tool.
Looks like you found the product market fit. There are enough people willing to pay for it. At this point, I'd suggest you should concentrate more on your go to market efforts.