If you switch between languages (in my case switch to Portuguese and then to German) the error message is in the language before. Also, if you switch to uselang=qqx, you don't see the message key. I switched after German back to English but the error message was still in Portuguese. After reloading then, the message was in English. But usually, reloading the page didn't help.
Lucie raised the priority of this task from to Needs Triage.
Lucie changed Security from none to None.