Another complete nonsense. I only speak Mandarin, and can communicate with people who only speak Cantonese if we both speak (really) slowly

I suspect you have some exposure to Cantonese, then. My experience in discussing this with many native Chinese speakers is that Mandarin, Shanghainese, Cantonese, and Fujianese are all mutually unintelligible. And not just slightly so, but as difficult to convey understanding as I (an English speaker with a touch of French) experience in Mexico.

But don't just take my word for it:

Chinese or the Sinitic language(s) (汉语／漢語 Hànyǔ; 华语／華語 Huáyǔ; 中文 Zhōngwén) is a language family consisting of languages which are mostly mutually unintelligible to varying degrees. [1]

(The above quotation bears this footnote: David Crystal, The Cambridge Encyclopedia of Language (Cambridge: Cambridge University Press, 1987) , p. 312. "The mutual unintelligibility of the varieties is the main ground for referring to them as separate languages." )

As you suggest that I may be prejudiced for political reasons, I respond that in this subject I have absolutely no bias. And let me add that the very same criticism could be directed at you.

A final criterion in differentiating language from dialect involves a language's political status, a factor that is external to the form of the language and sometimes even at variance with the culture of the speakers. Do the political authorities in a country consider two language forms to be separate languages or dialects of a single language? Extremely different, non-mutually intelligible language forms may be called dialects simply because they are spoken within a single political entity and it behooves the rulers of that entity to consider them as such: this was the case with Ukrainian and Russian in the days of the Russian Empire, where Ukrainian (called Little Russian) was considered a substandard variety of Russian (called Great Russian). This could also be said to be the case with the so-called dialects of Chinese in the People's Republic of China.

On the other hand, language forms that are quite mutually intelligible can be considered separate languages also for purely political reasons. Such is the case with Serbian and Croatian in the former Yugoslavia. Linguistically, these two language forms are more similar than the English spoken in Texas and New York; linguists, in fact, usually called them both by the name Serbo-Croatian. However, for entirely political reasons the Serbs and the Croats have deliberately invented separate literary standards to render their language more divergent than it really is. Furthermore, the Croats, being Catholics, use the Latin alphabet, while the Orthodox Serbs use a version of Cyrillic. A similar situation pertains is other cases, notably Hindi/Urdu, and Bengali/Assamese. [2]

Finally, you note that Mandarin is what is taught in schools, used in movies, etc. This is true for most of the PRC, but is not true for Hong Kong, Taiwan, and (I think) Tibet. But I don't see what it proves in any case. I will add, though, that in Chinese movies I frequently see Chinese subtitles, presumably put there so that those used to different "dialects" can understand.

[1] http://en.wikipedia.org/wiki/Chinese_language

[2] http://pandora.cii.wwu.edu/vajda/ling201/test3materials/dial...