Mandarin Chinese is called Guoyu or Hanyu. Hanyu is the national language and standard dialect in mainland China. It’s often called Putonghua, Zhongwen or Zhongguohua.
普通话 Putonghua
国语 Guoyu
汉语 Hanyu
中文 Zhongwen
中国话 Zhongguohua
汉字 Hanzi
In Hong Kong, Singapore, Taiwan, and other Chinese communities outside of mainland China, Mandarin is often referred to as Guoyu and is spoken alongside local dialects (just as in mainland China). The written characters (Hanzi) always remain the same.