Related is Prismatext, which intermixes the new language into the text of a book in your native language, so you get in-place, contextual learning of the new language:
I built something like this for my own personal language studies about a decade ago called "Kindle Swap". You fed it an ebook in ePub/MOBI format along with a list of known vocabulary and it would sprinkle them into the book so you could read it on your Kindle at your leisure later.