Siebrand made a list of the 25 biggest Wikipedias and their localisation. It is an interesting list and, as with all such statistics it shows the moment. The Polish for instance are currently working hard while there is not much activity for Spanish.
What I find amazing is that Esperanto and Volapük do better then Turkish. Danish is doing fine for the MediaWiki messages but messages for extensions need a lot of work.
Being one of the 25 biggest, I think that there should be a sense of "noblesse oblige". All the work done to create the content needed to be a top 25 Wikipedia deserves that the user experience is perfect. The localisation is part of this.