> I don't understand: is the 's' here for plural, or the abbreviation for "second"? Either way, it probably won't work well for other languages.

Fixed. See [06_video_progress_slider.js#L267](https://github.com/edx/edx-platform/blob/anton/video-i18n/common/lib/xmodule/xmodule/js/src/video/06_video_progress_slider.js#L267)