One of our goals at Google is to give everyone the information they want in the language they speak. We've been hard at work making Google products available in as many languages as possible. Recently we launched Google Calendar in eight more languages, bringing our total number of supported languages to 38 (and closing in on Gmail's 50). The new languages are Latvian, Romanian, Filipino/Tagalog, Serbian, Ukrainian, Bulgarian, Hindi and Indonesian.
To use Google Calendar in your preferred language, just sign in, click Settings in the upper right hand corner and look for Language.