Well, if [[:commons:MediaWiki:Gadget-HotCat.js]] wasn't working, there would have been lots of on-wiki complaints. I don't see any, so I presume it is actually working, just not for you.

It evidently works at the Commons, and at the English, Italian, and German Wikipedia.

Which Wiki, and what exactly "isn't working"? What's your username at that wiki?

Indeed. Are you using your own version, rather than the gadget?

While the Spanish Wikipedia does have the Commons version of HotCat set up as a Gadget (using hotlinking), it appears to me after studying the extensive JavaScript employed by https://es.wikipedia.org/wiki/Usuario:Ezarate that he actually uses

https://es.wikipedia.org/wiki/Usuario:Axxgreazz/Monobook-Suite/hotcats.js

That one appears to be a very old fork of Magnus' original code. It's completely unrelated to the Commons' HotCat.

Esteban, if hotcats (with "s") doesn't work for you, take it up with the maintainer at the Spanish Wikipedia. (Though User:Axxgreazz seems to be inactive.)

...and his user JS uses document.write() to include scripts.