Steps to replicate the issue (include links if applicable):
What happens?:
Labels and messages appear weirdly
What should have happened instead?:
It should show English labels
Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):
Other information (browser name/version, screenshots, etc.):
Why do you think this should work? https://www.wikifunctions.org/view/fr/Z20000 or https://www.wikifunctions.org/view/pa/Z20000 work as planned.
Ideally it should fallback to a language, but you can't really find this unless you are looking for it (I was :-)), so of course it doesn't really need to be done.
Change #1098005 had a related patch set uploaded (by Máté Szabó; author: Máté Szabó):
[mediawiki/extensions/ReportIncident@master] api: Update server-side processing for new IRS flow