Twitter is now available in Spanish. My rudimentary Spanish skills were able to detect this announcement posted on the Twitter blog - and yes, in Espanol. Ok, it helped that I saw the headline in English before I read the news in my RSS reader, but I digress.
I could deduct from the official Twitter post that they were very grateful to their translation volunteers for helping them to push out this update.
Also, if you go to your Settings link on Twitter.com, look for the language menu and select Spanish if you want this update.