When visiting any volcanic area, and Mount Merapi in particular, understand and respect the danger. Never take unneccesary risks.
Road safety is a large issue here and visitors are not advised to drive a rental vehicle, but to employ a local driver.
by train
The region is well served by trains with two main lines. The North Line runs along the north coast linking Solo, Semarang and Tegal, with stops in between. The South Line links Solo and Yogyakarta with small towns on the south coast. There is also a north-south cross island line in the west of the region, passing through Purwokerto.
The loop line in the northeast of the region linking Semarang with Jepara and Kudus is long disused despite still being shown on many maps.
talk
Indonesian Bahasa Indonesia is the official language of education and business. The majority of Central Java's population also speak Javanese, a related but mutually incomprehensible language. English and sometimes other European languages are widely understood in the major tourist areas.
by car
Hiring your own car is the most convenient way to cover some of the large distances between reactions in the region. Even by Indonesian standards though, driving practices in Central Java will scare most visitors. You will enjoy your visit more and certainly be much safer, if you hire a car with driver.