There's a new Spanish localization file, ui.sp.txt, that's been added to the ui.zip localization file download, and also attached to this post. Kudos to Luis in Spain for doing this all new, comprehensive translation.
Three or four tags were overlooked and will be updated soon. But in the meantime, this is such a step up from the prior partially-localized file that I thought it best to post right away.
As always, it would be great to hear comments / corrections / improvements from native Spanish speakers.