Even keeping up the localisation of MediaWiki is hard work. This is illustrated best by the languages that have full localisation and maintain their status. In the past ten days some 250 edits were needed just to keep up... These are the changes of the French Wikipedia, the changes of the Norwegian, Dutch, German and Arabic show a similar story.
In a way these are the lucky languages; for many languages it is a dream to get to the stage where all the messages are visible. In the last month a lot has happened for the Spanish and the Romanian language; Spanish is now approaching 100% of the MediaWiki core messages and Romanian has reached this milestone :)
In order to get to the stage of full localisation, the amount of maintenance increases as well. It is considered too much work for a single person to do and more people working on the localisation and proof reading will improve the quality the most.