I always compare your site to LingQ.
I study Mandarin and Cantonese. The ability to a) not have unnatural spaces between the words (there are no spaces in those languages) and b) easily link and unlink words are huge advantages over LingQ.
However I have similar issues with the app. If you make it so NO downward scrolling is necessary at all, just go to the next 'page' that would make your app better than the competition in my opinion.
Also, when browsing the library, get rid of all the flush-just show the title. Because currently it includes a picture and unknown/learning/known/total words. Together that means I can see only one item on my screen at a time.