The official language of Indonesia is Bahasa Indonesian. Across the islands however, there are over 700 recognised languages.
Whilst English is spoken in the bigger cities, and is used as a teaching language at institutions, you will not find an advanced level of English outside of these major cities. Before you move to Indonesia, it is worth learning some basic phrases so that you can get around in your first few weeks. When you have spent some time in the country, you will find that you have picked up more of the language than you expected to, make sure to utilise that skill and communicate with the locals and other international students.