Seems like this has been up for a while, but I have never seen the Google Request for Volunteer Translators before. You can sign up quickly, all you need is a Google account. Lots of languages are a 100% complete, see the language status page for languages that remain to be translated. Even if a language is 100% translated, it is possible that they add more "messages to the Google interface" according to the FAQs. Of course to qualify, you must follow the style guide and agree to the "Legal Stuff" as they call it.
Forum discussion at WebmasterWorld.

Comments:
manisha shah-ryan
02/23/2009 11:21 am
I speak and write in english hindi marathi gujarathi----i was looking for a volunteer position with google mountain view office
worldsoul
10/31/2009 06:42 pm
iam arabic-english translator,got ba in english . i think and feel that this should be one unified world as god create us with one word.so i like to help.
Elena
03/05/2010 07:15 pm
Hi, I would love to help google with its translations. I am a Spanish native speaker with already some translation experience. Looking forward to hearing from you soon, Elena M.
krisnan
09/15/2012 06:53 am
hi, I'm a Filipino and I notice that there's no voice to listen for the pronunciation for Tagalog so i would like to volunteer :) thanks!