The only official language is Indonesian, known in that language as Bahasa Indonesia (not Bahasa, which literally means “language”). They are similar to Malay ( pronounced in Malaysia, Brunei and Singapore), meaning that speakers of both languages can usually communicate with each other. Major differences are in loan words: Malaysian is more influenced by English, and Indonesian has been influenced more by Dutch. Written phonetically using the Latin alphabet and with a fairly logical grammar, Indonesian is generally considered one of the easiest languages to learn. Indonesian spelling is very regular and pronunciation is easy, especially for Japanese (except for the letter “l”), Italian or Spanish.
While Indonesian is the official language throughout the archipelago and spoken by almost all Indonesians, over 80% of Indonesians have their own ethnic language, with Javanese and Sundanese being the most commonly spoken. Some of the ethnic words make up the Indonesian language, so it’s usually a good place to start. If you stray off the beaten track, it’s a good idea to learn a few words of the local language to get on well with the local society. A number of ethnic Chinese communities are still speaking various Chinese dialects, especially Hokkien in Medan and Teochew in Pontianak.
Colloquial and slang Indonesian usually omits any indication of time and tense (of which there are few), prepositions and auxiliary verbs, and a sentence may consist of only one or three words. Often, because of the lack of clarity, additional questions need to be asked (especially with regard to whether an event has already happened, is happening now or will happen in the future) and local dialect loan words can confuse things further. If you use English, these tendencies carry over into their English because they translate from their slang into English, so you can experience the same problems – or worse.
In contrast to its neighbor Malaysia or the Philippines, English is generally not widespread. Staff in better hotels and airline staff generally speak an acceptable level of English, and English is widely spoken on the tourist island of Bali. Although English is a compulsory foreign language in Indonesian schools, you should expect only a basic to moderate level of proficiency.
A few educated seniors (70 years/older) in Indonesia may speak Dutch, but nowadays English is much more useful. Although Arabic is not widely spoken, many educated Muslims, especially those who graduated from Islamic religious institutes, understand Arabic to some extent, and many Arabic loanwords are found in Indonesian.