I have a premium subscription, but the only reason I got it was that I wanted the product to not disappear :) The features you get with the premium subscription would not have been a reason to upgrade for me at all. I don't know if this is a problem (I used to feel that way about Evernote as well), but maybe something to consider.
I even made a prototype (roughly two years ago) that looks _a lot_ like Readlang. I wasn't motivated to continue because I mostly wanted to learn French, not build this app, none of the apps I tried (Duolingo comes to mind) worked well with the way I like to learn.
This is even better than my fantasy, the ability to upload ebooks, for example :D Great job. You have my subscription! Good luck!
One question - do you translate many multi-word phrases? If you do then you would find the free plan restrictive. If you mainly translate single words then the free plan is probably fine.
May consider restricting the free plan a bit more in future. For now I'm happy to have a lot of free users to help spread the word and grow.