InterSub is a browser extension that transforms video watching into language learning. It adds interactive dual subtitles to popular streaming and education platforms, showing your native and target languages side by side. When you encounter unfamiliar words, simply hover to get AI-powered instant definitions and context translations. All looked-up words are saved to your personal wordbook, which you can sync with Anki or other apps for spaced repetition. InterSub supports over 30 languages and works on YouTube, Netflix, Prime Video, HBO Max, Crunchyroll, TED, Coursera, Udemy, LinkedIn Learning, and more. Additional features include collocation detection for English phrasal verbs and idioms, remote control from your phone, and a 'show subtitles on pause' mode. The extension is free with premium plans available.