There are around 900 million native Mandarin speakers in the world, only some 70 million Cantonese speakers. Within that mass of Mandarin speakers there are still many dialects which are very hard to understand for speakers of standard Mandarin -- something close to the dialect spoken around Beijing. This standard is also the basis for the national language of Taiwan, where I live. The major differences between what you hear on the streets of Taipei and Beijing are in accent and, in some instances, slight shifts in vocabulary.
More distant dialects may have words that are never heard in standard Mandarin which sometimes derive from different grammar.
In Crouching Tiger, Zhang Yi, the wayward young swordfighter speaks Mandarin with a distinct north Chinese accent. The dashing Xinjiang chief is played by a Taiwanese actor and speaks with a Taiwanese accent. The other two leads -- Michelle Yeoh and Chow Yun-fat -- speak fluent Mandarin but with Cantonese accents.
Once you leave Mandarin behind, things are harder still. The different Chinese languages, such as Mandarin and Cantonese are as distinct as, say, Italian and Spanish, though the Chinese authorities, ever keen to emphasise unity, tend to refer to them all as dialects.
So its not true that the written form of Mandarin is universally comprehensible -- though a Cantonese speaker will get the gist of something written in Mandarin. Cantonese, and all the others, has unique characters for words it has but others don't.
Until the early years of the last century, classical Chinese with its own completely separate grammar and vocabulary, was used for all official writing and so formed a bridge between speakers of different languages. Vernacular writing was only used for less well-regarded things such as popular literature and the prompt books for marketplace storytelling. That changed in the 20th century when a standard written language representing the spoken forms was developed, catching on quickly with the drive for modernization.
Since most emigration stemmed from south China, well-established overseas communities tend to speak Cantonese or another southern language. That's changing though as more and more Mandarin speakers have left to settle among the barbarians.
And then there are several non-Chinese languages spoken widely by non-Han Chinese some of which have several million speakers, but many of which are being smothered by the spread of Mandarin. Here in Taiwan, there are the vestiges of several languages spoken by the Aboriginal communities which are far distant from anything found elsewhere in the world.
