Hoi, We have developed an extension called LocalisationUpdate. What it does is load those localisations that are an exact match between the message on your system and the message on SVN in English.
In theory it can be made to work for older releases, in practice it will work from release 1.15 onwards. I have been testing it for some time now and it works for me, what we want to know is that it works for you.
We are looking for people who run a MediaWiki Wiki and are willing to upgrade to release 1.15 and test LocalisationUpdate in their environment. We will support the first five wikis for five different languages that are currently not fully localised. Obviously, in order to test it, you will have to do some work at translatewiki.net as well.