OK, just to clarify:
The written Chinese language is more or less constant everywhere, except that there two systems of characters: traditional (used in Hong Kong and Taiwan) and simplified (China, Malaysia and Singapore).
The spoken Chinese language however has many variations. The major 'dialect languages' are:
Mandarin (the official spoken language),
Cantonese (Hong Kong and southern China),
Shanghainese (Shanghai),
Hakka (some parts of southern China),
Taiwanese (Taiwan),
Hokkien (Fujian)
and Teochew (Chaoshan region).
In essence, spoken Chinese has many 'dialect languages', since they are not 'dialects' as we understand the term in, for example, English. The differences are so vast across the spoken medium that they qualify as separate languages, which they are. Comparing 'dialect languages' would be like comparing spoken English to spoken French (with the exception of the last three, which can be similar), so two speakers of different 'dialect languages' would not understand each other at all. Structure-wise, they are also very distinct; Cantonese is Subject-Verb-Object like English, whereas Shanghainese is Subject-Object-Verb like Japanese. These 'dialect languages' are not merely pronunciations of the written language, character by character (except Mandarin); they are learnt separately and do not correpond to the text completely.
Thus, there is one written Chinese language and many different spoken Chinese languages.