Since most of the places you mentioned are remote area, people there always speak local dialect. But they will speak mandarin to people from other cities or countries. Also, there is no starting fare on the cab meter in these remote places, you will need to discuss the price with driver before you take it. Therefore, if you do not have a guide, I suggest you ask more drivers or ask the staff at hotel to help you. As for Xining and Lhasa, the starting fare on the cab meter is around CNY10. By the way, foreigners are not allowed to explore Tibet alone without guide according to the policy.