Twitter has recently crossed its milestone of 100 Million users as they mentioned in their blog. And alexa indicates Twitter still has a major traffic coming from United States (27.7%), and other countries are like Japan (7.7%), India (6.1%), UK (5.5%), Brazil (4.1).
So may be in order to attract more users from countries other than US, Twitter announced they will be supporting 17 more languages. And Hindi is one of them, after launching Hindi, Filipino, Malay and Simplified and Traditional Chinese in next weeks.
Like other languages, you [will] be able to access twitter in Hindi from this url http://twitter.com/?lang=hi.
Twitter will be first social network to officially support Hindi as language. With India they might be targeting another chunk of million users out of 100 Million online users of India.