Here's my first draft for the Spanish translation. Please note that:
It's fairly complete, but not quite ready for production. Spanish is not my mother tongue (I'm French), but I've been living in Spain for 10 years so it should be acceptable. Contributions and suggestions are welcome. I will post updates as necessary.
This is Spanish for Spain. Some parts of the translations might not be appropriate in South American countries for example. In particular, I've used the less formal, more direct version of "you": "tú" as opposed to "usted", which is much more common in Spain.
I love ProcessWire and I'm glad I can contribute to this awesome project.
UPDATES:
December 10, 2013: Translation files updated for dev version 2.3.8 (see "spanish-spain-december2013.zip" below)
March 4, 2014: I've created a Github repository that I plan to update whenever possible. All updates will be posted there from now on. June 16, 2014. Update to 2.4.4.
spanish-spain.zip
spanish-spain-december2013.zip