Thanks a lot. Have a good day! 
 What is the official language? 
 One more question, I don't understand what you mean by human development index and what causes Tibet to be ranked the lowest? 
 Linguist generally classify the Tibetan language as a Tibeto- Burman language. 
 I am sorry I do not have any further information as to what causes that but another interesting fact about economy is from January 18-20, 2010 a national conference on Tibet and areas inhabited by Tibetans was held in China and a substantial plan to improve development of the areas was announced.
Linguist generally classify the Tibetan language as a Tibeto- Burman language.