The proportion of readers who cross the divide to become editors is vanishingly small. If we want to preserve or increase it, we should fret about making the account creation interface as easy to understand as possible. It's especially important to speak plainly and avoid jargon.
The message 'centralauth-account-exists' (en: "Cannot create account: The requested username is already taken in the unified login system.") reveals more about the software implementation than is sensible to reveal to a prospective user -- especially one who just had his or her desired username rejected. We should just use userexists.
Previous discussion here: https://en.wikipedia.org/wiki/MediaWiki_talk:Centralauth-account-exists.