Noted during some testing at Translatewiki in a German interface. A code like:
var t = mediaWiki.msg.get('pagecategories', 6);
alerts with "<pagecategories>" (same for other existing messages). It doesn't seem to be undefined, otherwise it would have alerted me "undefined", so it probably didn't get that the page exists (http://translatewiki.net/wiki/MediaWiki:Pagecategories/de).