The official language is Mandarin Chinese (Putonghua), however numerous dialects are also spoken throughout the mainland. Among these dialects, large groups speak Cantonese, Fukienese, Xiamenhua and Hakka. Cantonese is the main language of the Hong Kong and Macau SARs; whereas Mongolia, Tibet and Xinjiang (autonomous regions) have their own languages.
English is not widely spoken, therefore a phrase book would be extremely useful to point to key phrases written in Chinese characters. Another useful item would be a map with place names written in both English and Chinese.
You may want to use your international cellular phone rental to locate a translation agency when visiting china.