About [[MediaWiki:Autosumm-new]]
Why isn't this message used in Translatewiki for the edit summary of created pages (example), like in other wikis?
Because of the different use case we have here. Most of the edits made on translatewiki.net are new pages because of the nature of the work done here. Having that default edit summary doesn't really help in Special:RecentChanges, so we disabled it.