Well, I wanted to help and try to help MediaWiki by translating the responsible parts of the javascript into German. I've downloaded the LanguageDe.php, cause i thought there i would find this part of the JS. But i can't find it.
The message these are defined in is, I believe, "monobook.js" (global JavaScript for MonoBook skin). There are also some messages called "tooltip-..." and "accesskey-...", but these are possibly no longer used (if this can be confirmed, they should be removed from LanguageXX.php, really). So you were looking in exactly the right place, I think.
See also http://de.wikipedia.org/wiki/MediaWiki:Monobook.js, for the German Wikipedia's version (and http://de.wikipedia.org/wiki/Spezial:Allmessages for possible other translated messages)
[PS in reference to a completely different thread: interesting, the quoted linebreaks seem to have broken here; GMail using not-so-clever web forms, I guess...] -- Rowan Collins BSc [IMSoP]