Some key interface messages seem to have disappeared on the account creation form. Specifically, MediaWiki:Createacct-helpusername and MediaWiki:Createacct-imgcaptcha-help are both gone.
Checking English Wikipedia and MediaWiki.org with uselang=qqx shows that these messages are still in the code, but are not being displayed for some reason.
This is really bad, since Createacct-imgcaptcha-help links to the local help pages for users who have accessibility issues and can't read the CAPTCHA.
Perhaps the transition to JSON screwed something up here?