Yeah, the JS strings haven't been wrapped in gettext anywhere yet (except a few places in khan-exercises), but they will be before we release an i18n version.

For these errors, I think it would be good to be able to wrap the "intro" text (e.g. "Error checking subtitles progress"), and then leave the actual error message, sent from the server, untranslated. I like the idea of having something along the lines of "please send a report to the admins". Perhaps we want to look into having them be able to click a button to save a traceback and sync it back to the central server when available, with report text ("what were you doing at the time")?