Last modified: 2014-04-14 19:50:14 UTC
See https://www.wikidata.org/wiki/Special:Tags on Wikidata and sort it by the third column. The first, second, third and fourth tag have the third column empty but their MediaWiki: pages do exist. (Best viewed when you as sysop and can see "(edit)" links.) I think it's because the tags include '/' symbol.
Yeah, slashes in tag names (and thus in names of MediaWiki: namespace pages) are probably impossible to support due to how our internationalization system works. I'm not sure what we could do about it, maybe map '/' to '-' before looking for a message or something? (So the page for a tag called "a/b/c" would be "MediaWiki:A-b-c".)