That message is clearly malformed--no other language appears to use {{GENDER}} in this message, where I have seen GENDER used in the past it has been in ALL CAPS, and I'm pretty sure there has to be an argument to the magic word (so "{{GENDER:$1|something|something else}}" rather than "{{GENDER:|something|something else}}").

If I'm wrong on any counts, please add more info! :)

(note: there are a bunch of other messages in the same file with much the same syntax, so either it's very broken or I'm very wrong)

https://translatewiki.net/wiki/Gender#Testing_and_more_examples

"Your gender is {{GENDER:|male|female|unknown}} → unknown (should be unknown. This feature does not work in page content. Should it? Put your comments on talk. )"

Apparently this is even worse when using it in JavaScript, since the l10n features, however ugly, are even uglier on the JS side.

Fixed with https://translatewiki.net/w/i.php?title=MediaWiki%3AMwe-upwiz-deeds-macro-prompt%2Fcs&diff=4172980&oldid=3248756. Also fixed for language "rue".

Two languages used GENDER in a message it was not supported in. A new issue
was reported for this (bug 39559). This solves the immediate issue. The JavaScript parser should ideally not b0rk over this. We've had a similar issue in UploadWizard before.