The city weather is hot and humid. Cotton clothes are recommended, as are T-shirts and knee-long trousers. Some places like the mosques (masjid) and temples require polite dress such as long trousers/skirts. As Indonesia is still fairly conservative, wearing hot pants/mini skirts will attract attention and might be considered impolite.
Semarang is off the tourist trail and knowing basic Indonesian will be helpful, although younger people can often speak some English.