You ask a very good question. The different regional speech varieties of Han people in China would be called "languages" in a context of Roman-descended Europeans, and they should be called "languages" for clarity in discussing issues of language policy. My wife is from Taiwan and grew up speaking the southern Min language as it is spoken on Taiwan (that is, "Taiwanese") and was also forced to learn Mandarin in school. Taiwanese and Mandarin are at least as different, linguistically and practically, as English and German. I have learned some of each of those languages, and also some Cantonese and some Hakka. Those are all different (to be sure, cognate) languages, and a speaker of one of those languages cannot understand a speaker of the others.

A linguistic survey conducted by the P.R.C. regime discloses that barely more than half the population of China is conversant in the standard form of Mandarin.[1] Of course younger persons rather than older, and urban persons rather than rural, are more likely to be able to communicate with one another in standard Mandarin ("Putonghua"), but there are many people who have a need to communicate by speech who cannot. Contrary to much myth about the issue, I have NEVER seen two Chinese persons who cannot speak a mutually comprehensible language resort to writing out Chinese characters for one another in an attempt at conversation. To the contrary, I have seen many more examples of Chinese people (some from one place, some from another in China) speaking to one another in English as a useful interlanguage, although these days most people who can do that can also use Putonghua as an interlanguage.

Chinese characters still represent SPEECH (not ideas or abstract concepts) and do so in a way that is specific to the particular Chinese (Sinitic) language that one speaks. The long story about this can be found in the books The Chinese Language: Fact and Fantasy[2] or Visible Speech: The Diverse Oneness of Writing Systems[3] by the late John DeFrancis or the book Ideogram: Chinese Characters and the Myth of Disembodied Meaning[4] by J. Marshall Unger. The book Reading in the Brain: The Science and Evolution of a Human Invention[5] by Stanislas Dehaene is a very good book about reading in general, and has a good cross-cultural perspective.

I'll give an example here of how Chinese characters reflect speech more than they reflect meaning-as-such. Many more examples are possible. How you might write the conversation

"Does he know how to speak Mandarin?

"No, he doesn't."

他會說普通話嗎？

他不會。

in Modern Standard Chinese characters contrasts with how you would write

"Does he know how to speak Cantonese?

"No, he doesn't."

佢識唔識講廣東話？

佢唔識。

in the Chinese characters used to write Cantonese. As will readily appear even to readers who don't know Chinese characters, many more words than "Mandarin" and "Cantonese" differ between those sentences in Chinese characters.

Many other examples of words, phrases, and whole sentences that are essentially unreadable to persons who have learned only Modern Standard Chinese can be found in texts produced in Chinese characters by speakers of other Sinitic languages ("Chinese dialects"). Similar considerations apply to Japanese, which is not even a language cognate with Chinese, and also links Chinese characters to particular speech morphemes (whether etymologically Japanese or Sino-Japanese) rather than with abstract concepts.

[1] http://news.xinhuanet.com/english/2007-03/07/content_5812838...

[2] http://www.amazon.com/Chinese-Language-Fantasy-John-DeFranci...

[3] http://www.amazon.com/Visible-Speech-Asian-Interactions-Comp...

[4] http://www.amazon.com/Ideogram-Chinese-Characters-Disembodie...

[5] http://readinginthebrain.pagesperso-orange.fr/intro.htm