Punakha Dzong was created in 1638.

Thimphu was built by the late king Jigme Dorje Wangchuk, to replace the ancient capital of Punakha.

The major cities in Bhutan are the capital, Thimpu, the former capital, Punakha and the city with the country's only airport, Paro.

Dzongkha is the national language of Bhutan, so any city in Bhutan would speak it, such as:

Thimphu

Damphu

Jakar

Mongar

Paro

Phuentsholing

Punakha

Samdrup Jongkhar

Trashigang

Trongsa

I am not exactly sure of any accurate statistics on this topic, but I can tell you that Dzongkha and its dialects are the native tongue of eight southern districts of Bhutan: Phodrang, Punakha, Thimphu, Gasa, Paro, Ha, Dhakana, and Chukha). Also, Dzongkha study is mandatory in all schools in Bhutan.