Mothertongue

I built Mothertongue so I could watch English tech talks in Vietnamese. It's an open-source macOS app that dubs a talk into spoken Vietnamese and keeps the voice in time with the speaker. Most good talks about building software and startups are in English. A lot of Vietnamese students and developers watch them with subtitles they can't keep up with, or don't watch at all. Auto captions read like a translation and often mix up who is speaking, and a dub that drifts ahead of the speaker is hard to follow. You paste a YouTube link or a whole HackerTube category. The app finds the speakers and reads the whole talk once to write a brief: the topic, the tone, each speaker's gender and the Vietnamese pronouns they should use, and which product names stay in English. Then it translates each line into spoken Vietnamese that fits the seconds the speaker takes, rewrites lines that sound translated or pile up filler words, and gives each speaker a southern Vietnamese voice picked by gender and speaking pace. English and Vietnamese subtitles sit side by side, and one click switches back to the original audio. On a 28-minute talk, lines where the dub went quiet 4 or more seconds before the speaker dropped from 54 of 89 to 4 of 257 (the line count changed when I moved to word-level timing). That run cost $0.30, about one cent per minute of video, and the app shows the cost for every video.