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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

This seems useful for related languages, but even then I wonder whether it is a good idea to blur the lines required for code switching.