Theravāda (Pali, literally "school of the elder monks") is a branch of Buddhism that uses the Buddha's teaching preserved in the Pāli Canon as its doctrinal core. The Pali canon is the only complete Buddhist canon which survives in a classical Indic Language, Pali, which serves as the sacred language and lingua franca of Theravada Buddhism. Another feature of Theravada is that it tends to be very conservative about matters of doctrine and monastic discipline. As a distinct sect, Theravada Buddhism developed in Sri Lanka and spread to the rest of Southeast Asia. 

Theravada also includes a rich diversity of traditions and practices that have developed over its long history of interactions with varying cultures and religious communities. It is the dominant form of religion in Cambodia, Laos, Myanmar, Sri Lanka, and Thailand, and is practiced by minority groups in India, Bangladesh, China, Nepal, and Vietnam. In addition, the diaspora of all of these groups as well as converts around the world practice Theravāda Buddhism. Contemporary expressions include Buddhist modernism, the Vipassana movement and the Thai Forest Tradition. 

Theravāda Buddhism is followed by countries and people around the globe, and is: 

Today, Theravāda Buddhists, otherwise known as Theravadins, number over 150 million worldwide, and during the past few decades Theravāda Buddhism has begun to take root in the West and in the Buddhist revival in India.
What language is it in?
The Pali canon is the only complete Buddhist canon which survives in a classical Indic Language,