A bit of a late answer, but your idea is pretty cool, definitely something I could see myself using if it were integrated in an ebook reader or something simila

A bit of a late answer, but your idea is pretty cool, definitely something I could see myself using if it were integrated in an ebook reader or something similar. For German (and a few other languages, actually), a very good website to fetch sentences and translations from is Linguee. I might implement something similar on top of epubjs-reader.