Mandailing or Batak Mandailing is an Austronesian language

Austronesian languages

The Austronesian languages are a language family widely dispersed throughout the islands of Southeast Asia and the Pacific, with a few members spoken on continental Asia that are spoken by about 386 million people. It is on par with Indo-European, Niger-Congo, Afroasiatic and Uralic as one of the...

 spoken in Indonesia

Indonesia

Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...

, the northern island of Sumatra

Sumatra

Sumatra is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia , and the sixth largest island in the world at 473,481 km2 with a population of 50,365,538...

. The language pertains to the Malayo-Polynesian of Austronesian languages.
It is written using the Latin alphabet

Latin alphabet

The Latin alphabet, also called the Roman alphabet, is the most recognized alphabet used in the world today. It evolved from a western variety of the Greek alphabet called the Cumaean alphabet, which was adopted and modified by the Etruscans who ruled early Rome...

 but historically used Batak script.

Sources

Adelaar, Alexander, The Austronesian Languages of Asia and Madagascar: A Historical Perspective, The Austronesian Languages of Asia and Madagascar, pp. 1–42, Routledge Language Family Series, Londres: Routledge, 2005, ISBN 0-7007-1286-0 Siregar, Ahmad Samin, Kamus Bahasa Angola/Mandailing Indonesia, Jakarta: Pusat Pembinaan Dan Pengembangan Bahasa, Departemen Pendidikan Dan Kebudayaan, 1977.