Huh, I literally built an app that does what you describe! Basically an e-reader for language learners called Polyreader. It's a stand-alone app, not a browser

Huh, I literally built an app that does what you describe! Basically an e-reader for language learners called Polyreader. It's a stand-alone app, not a browser plugin, but it has lots of additional features like in-line translation. Maybe I should finally open-source it!