Reason is that something loads mediawiki.ui.input module on that page. You can confirm this yourself by typing mw.loader.load( 'mediawiki.ui.input' ); on working wiki.

The gadget module ext.gadget.pope loads 'mediawiki.ui.input' .
But apparently, as per https://pl.wikipedia.org/wiki/MediaWiki:Gadget-pope.js The gadget is supposed to be intiated in some special pages and not in Special:CX.

That page looks equally broken for me on all wikis I checked. What should I be looking for? Are you talking about Special:ContentTranslationStats?

This is what we see in plwiki. See the 3 tabs appearing with large widths

Expected rendering, as seen in https://en.wikipedia.org/wiki/Special:ContentTranslation

Oh… I was confused because Special:ContentTranslation was claiming that no such page exists, I filed T125306 about it.

I also filed T124896 about how ContentTranslation's lack of sidebar causes another gadget to break, which in turn breaks ContentTranslation.

So… why is ContentTranslation adding 'mw-ui-input' to those tabs if it doesn't expect the .mw-ui-input styles to apply?…

Change 267443 had a related patch set uploaded (by Bartosz Dziewoński):
Do not add 'mw-ui-input' class to translation filter tabs (cx-filters)

https://gerrit.wikimedia.org/r/267443

Change 267443 merged by jenkins-bot:
Do not add 'mw-ui-input' class to translation filter tabs (cx-filters)

https://gerrit.wikimedia.org/r/267443