As reported by @Doc_James:

Having trouble getting subtitles to work. We have been translating the captions into other languages but unable to figure out how to get them to show.

They work but only right on commons https://commons.wikimedia.org/wiki/File:Pneumonia.webm

Supposedly the issue may be:

<div style="display: block; position: absolute; bottom: 15px; width: 100%; opacity: 0.8; text-align: center; font-size: 133.333%;" data-capid="141" class="track" lang="en"><span style="pointer-events: auto;" class="ttmlStyled"><span style="position: relative;">actually isn’t any exudate or fluid in the<br>alveolar spaces, and so essentially no consolidation,</span></span></div>

There’s a <div> element missing on the https://en.wikipedia.org/wiki/File:Pneumonia.webmpage.
specifically, there’s a div class=“videoHolder” that is supposed to contain another div class=“track”. The track is the element that is used to update the closed captions dynamically. If it is missing, the captions have to place to be displayed. The need to add that div in that page. Not sure who to go to to fix that. 7:54 PM
I tested a fix, and it works.

I can verify that I am not seeing the captions on desktop wikipedia, but I am seeing them on chrome.