Please note that this bug is a big deal for my IEG video project, and I imagine that it affects Fundraising and others who use video subtitles as well. I would like to triage this as high priority if that's OK.

--> TimedMediaHandler code.

Pretty sure this is the same problem as T122737 - at least the "network" tab of the Developer Tools in Firefox shows the same 404s due to the missing namespace in the URL. Hence merging as duplicate.

Little intermittently about it. It's plain broken, see latest tickets on T122737. I'm having trouble finding reviewers for the code, and have little time atm to go chasing after people and do follow up etc etc.

if a file is displayed on Commons and a language is selected in the "Language select" box instead of or in addition to using the CC button, the subtitles sometimes display.

They always display for me personally really. Only not Danish.. I suspect that that specific track is broken.

Also, it's confusing to have both a "Language select" box and a CC button which may be selected to conflicting languages.

The language select box is a local commons javascript. I don't think the player listens to it at all. (Nor should it).