A small tip: to find most MediaWiki: messages that haven't yet been translated, view the HTML source of Special:Allmessages and search the source code for the following string:

class="new" title="MediaWiki:

This will lead you to any redlinked pages in the MediaWiki namespace, and the trailing colon ensures that you skip over any talk pages. I found Niklas' "Internationalisation news" a lot more useful for finding out about recent interface changes, but if you find that your wiki has some catching-up to do, this is the way to go.