When you install the 1.15 stable version of MediaWiki on your system, you will only get the localisation available at the time of the release. After that you do not get any more regular updates. This is not because no more localisation work is done at translatewiki.net for the many relevant messages, it is just because there is no mechanism to get them to you in a timely manner.
The LocalisationUpdate extension will be a game changer; it will bring you all the latest localisations from Wikimedia Foundation's SVN. It will bring them based on the definition of the English message in SVN being EXACTLY the same as the local message on your wiki. So when you have a MediaWiki Wiki in a language that is in need of improved localisation support, you can choose to become part of the beta testing of LocalisationUpdate.
TheDevilOnLine is willing to support five Wikis in five different languages who want to test out this new functionality. To be eligible you will have to upgrade to release 1.15.