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 l

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. https://specularrealms.com/projects/kindle-swap https://specularrealms.com/projects/kindle-swap