Mandarin is national language of China, Taiwan, and some other countries such as Singapore. In a huge country like China, people from different region speak different dialect. Thus, to unity the people mandarin is used. Commonly for official papers, etc. About your question, traditional or simplified Chinese, they're the writing method. Traditional Chinese is the original character, meanwhile simplified Chinese is the simplified method (the word is self-explained). To make it simple, I'll show you the difference. The word "please" is written 請 in traditional Chinese and 请 in simplified Chinese (both are pronounced qĭng). Now, you can see that traditional Chinese has more strokes than simplified Chinese. Traditional Chinese is used in Hong Kong and Taiwan, simplified Chinese in China and Singapore.

Mandarin can be written in either simplified or traditional characters. It is not a writing system, it is a way of pronouncing the characters that is used both in Mainland China and in Taiwan. Mandarin is called a "dialect" of Chinese, as opposed to Cantonese, Shanghainese, Taiwanese, etc., though these can be thought of as separate spoken languages (they use the same written characters for the most part). They are not "dialects" the way other languages have dialects; speakers of different Chinese "dialects" cannot understand each other, so they usually use Mandarin to communicate with speakers of other "dialects".

Simplified is for mainland people and Traditional is for Hongkongers and Taiwaneses.

Simplified i think.

I see, thanks for the answer. But, when will it become traditional?