About this project

DualSub Replay is a free, open-source Android application for language learners who watch YouTube. It displays original captions and a translated version simultaneously, supports on-device translation into 59 languages via Google ML Kit, and lets users tap any subtitle paragraph to replay that exact moment. The app also includes saved-word vocabulary cards with spaced-repetition practice and progress tracking for immersion time. The app embeds the mobile YouTube website in a single WebView, so browsing, search, playback, comments, and recommendations remain available. It can retrieve manual or auto-generated captions and parse multiple caption formats. In landscape mode, users get a resizable split view with the video and subtitle timeline side by side. No account or API key is required, and translation happens locally on the device. DualSub Replay is distributed as an APK from GitHub releases and requires Android 8.0 or newer. The project uses an undocumented YouTube transcript endpoint, which may break if YouTube changes its internal API. The source code is MIT-licensed, while the app distribution follows GPL-3.0. Development includes automated CI builds, managed-device tests, and an optional F-Droid variant using Mozilla's Bergamot translation engine.