[siong1987]

Not just Italian, same applies to Mandarin too.

Mandarin speakers from different regions (Hong Kong, Taiwan, different parts of China, Malaysia, Singapore) speak in different accents but we usually don't have problem understanding each other (yeah, sometimes we make fun of the different accents).

[guspe]

It's awkward to compare Italian and Mandarin to the state of English in the world. English is a language that was "exported" to America by the British Empire. It was also "exported" to Australia and many other former colonies. It's no surprise, really, that these people have difficulties understanding each other. While China and Italy inhabitants share common borders, native English speakers share only the relationship to their former colonizer.

There are other languages that suffer from the same problem. Brazilian Portuguese differs greatly from Portugal's, which is also different from East Timor's. I don't know much about Spanish and French but I assume the situation is analogous.

It's important to note that such differences between colonies and their colonizers were once a strong argument towards independence.