Mandarin, (官话Guān huà, speech of officials, or 普通话 pǔ tōng huà, common speach), is the Chinese speaking language and native to mainland China, Taiwan, Singapore and some overseas communities. Because Mandarin originated in North China and most Mandarin dialects are found in the north, so sometimes it’s referred to as Northern Chinese.
Another commonly heard speaking Chinese is Cantonese which is used in Southeastern China, Hong Kong, Macau and some overseas communities. In addition to these two major dialects, China has other small dialects. The promotion of Mandarin is mainly to eliminate dialect barriers, to facilitate social communication.
Chinese is not equivalent to Mandarin. The written Chinese is called Chinese characters. A character almost always corresponds to a single syllable which is unlike alphabetic system. There are two forms of writing Chinese, traditional and simplified Chinese characters.
Traditional characters are used in Taiwan, Hong Kong and Macau. Simplified forms are used in mainland China, Singapore, and Malaysia.
The above is just a brief explanation of the confusion of Chinese. I think you or your client should be clear about what target group and its corresponding speaking language you need for your project on hand.