What to fix. The step "Describe" of the Upload Wizard at Commons gives you the option to add descriptions in multiple languages. This used to be a very long list. Now it contains a mere 11 languages.

Probably https://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/UploadWizard/resources/mw.LanguageUpWiz.js?&pathrev=85935&r1=85934&r2=85935

(thanks SPQRobin for pointing this one out)

Perhaps the LanguageHandler.js added in r85935 doesn't get run, or gets
run in the wrong order, possibly related to the 1.19 upgrade. Adding Brion as well since he's been looking at other UW loading issues.

https://www.mediawiki.org/wiki/Special:Code/MediaWiki/85935 was suspected but then there is https://commons.wikimedia.org/wiki/MediaWiki:LanguageHandler.js

r112253 fixes this on my local installation -- looks like a regression in r107641 broke the loading by turning the absolute URL into a relative URL.

I can confirm that with a local copy of Commons' MediaWiki:LanguageHandler.js I see a giant list of languages now.