[[MediaWiki:Newuserlog-create2-entry]]

MessagesEn.php has a comment "do not translate or duplicate this message to other languages", I guess translation should be disabled here?

The message key is already in $wgIgnoredMessages of MessageTypes.inc. I do not understand why the message is translatable :-/

Probably because it's still present in branch 1.18. Once support for 1.18 is dropped that will be resolved.