Learning with Texts allows you to copy and paste in a body of text, note which words within that text you do and don't know, quickly look up the words you don't, and create flashcards from them. It was designed with languages that use spaces in mind (to determine where one word stops and the next begins), and works quite well with them. For languages that don't use spaces, like Japanese, it's much more cumbersome (as some elbow grease will be needed to indicate where one word stops and the next begins) but it's still a useful tool. The interface is not very intuitive and there's definitively a significant learning curve to climb before you get your sea legs, but I'd recommend breaking out your climbing gear because the price is right ($0) and there's no other free tool that does the same thing.
In fact, the only place you can do the same thing that I am aware of (to the comments if you know of another!) is LingQ. However, LingQ only allows you to input 100 terms for free; from there, you have to subscribe to get more. While I've found that LingQ is a bit more user friendly and intuitive, it's hard to beat free.
My initial approach to reviewing Learning with Texts was to simply pick some article I was reading, throw it up there, run through the process with it, and then report back in the form of a review. However, the initial article I selected was in Japanese, and it quickly became apparent that the Learning with Texts experience is going to be vastly different depending on whether you're using a language with spaces, like all major Western languages, or a language without spaces, like Japanese. As such, I also decided I'd add the text of a short comment from my blog that was written in Portuguese to test out how it works with languages that use spaces.