What are your long-term plans with this? I'd love at some point to be able to combine something like this with an algorithm I'm working on called Guided Immersion.
Basically, the system tracks what words you know and don't know, and so could tell you how hard a given sentence is for you. And it also tracks what words it would be useful to review and/or learn (spaced repetition and frequency analysis), to tell you how valuable a sentence would be for you.
The algorithm is generic and can be adapted to any language; right now it's been adapted to Mandarin Chinese, Korean, and New Testament Greek. (Which unfortunately so far doesn't seem to overlap with any of your available languages.) I'm working on an API to allow any content providers to use the algorithm.
Adding this to your system could help focus the content you're showing people to things that they're likely to be able to understand without having to look up most words, and helping them incrementally grow and solidify their vocabulary using the built-in spaced repetition.
Drop me a line if you want to chat at some point -- my email is in my about.