Use common sense, as you would do in your own country. Most common is fortunately theft, violent crime is rare in Indonesia. However, it's wise to be extra cautious in Jakarta and other big cities See also wikitravel: Jakarta section. Avoid crowds.
Traffic can be chaotic. Watch out for trucks, cars and other moving objects. One rule applies: "might is right". Don't expect traffic to respect pedestrian "Zebra" crossings even when a police(woman is in sight). When crossing a busy street - be confident, make sure your intentions are clear to the oncoming drivers. Cars never stop, but if they know what they can expect from you, they will avoid you. Using hand signal like put one hand in the air or signaling stop might help, too.
talk
While Indonesian Bahasa Indonesia is the lingua franca understood by almost everyone, the majority of Java's population also speak Javanese, a related but mutually incomprehensible language. As the largest single language in Indonesia, Javanese influence on Bahasa Indonesia has been quite significant, and particularly in Central and East Java you'll even hear the local pronunciation change — the most notable change is a turning into o, so eg. Surabaya becomes Suroboyo.
Other significant local languages include Sundanese, spoken in West Java, Betawi in Jakarta and Madurese, spoken on and around the island of Madura.