As I mentioned in my review of the Malay phrasebook of Lonely Planet, it is important to select the correct one. I first purchased LOnely Planet's Malay, thinking that the same language was spoken in the Malay and Indonesian countries. Only by comparing the two, Malay and Indonesian,I discovered that, though one could get by in Indonesia using the Malay version, many words are different.
This is due to the Dutch influence during their colonisation, whereas the Malay version is heavily influenced by the English language due to the time of their colonisation in that region.
So, when travelling to Sumatra, Java, Bali and the other Indonesian islands, Indonesian is the best option.