Bahasa Indonesia is the official language of Indonesia. However, there are also 737 other languages spoken throughout the islands of Indonesia. Most everyone is fluent in Indonesian along with one or more local dialects.
This past week I have been working on a water tank with some Indonesian guys. And since I am studying Indonesian, it has been a great experience to hear mostly Indonesian while we have been working on the tanks (in Bali, you tend to hear mostly Balinese in conversations).
The interesting thing is that there were six guys working together and they were all from different places, each with their own local language. However, we all shared Indonesian as a common form of communication. After a while, everyone started wanting to learn phrases and terms from all the languages.
It went a little something like this: someone would ask for a saw, "di mana gergaji?" After that, the person using the saw would ask what the word for saw was in English. After hearing the answer, they would repeat it (it sounded something like "sew" when they repeated it). Then the Javanese guys would have to say it in their language, then the Balinese, and so on... So my head is currently spinning with words from Javanese, Balinese, Indonesian, and a little bit of Batak.