Lots of links on mw.org are of the form [[ Special:MyLanguage/Some page ]] for i18n.
Hovercards doesn't show a popup for these. It makes its API request, but gets back e.g.
"title": "Special:MyLanguage/Data and developer hub",
instead of useful information.
Either the Popups extension or the API query needs to "push through" Special:MyLanguage `HTTP/1.1 302 forced.302` to the page in the user's language. FWIW Popups' API request specifies `redirects=true`.
[[ https://www.mediawiki.org/wiki/Project:Sandbox?oldid=1430470 | This Sandbox edit]] shows the problem