Here are a few corrections on your english, hope you don't mind:
How much different are Australian, American and Britain languages?
How different are ...


Whether does an ununderstanding happen?
Do misunderstandings occur?

What do Britains think of the US and AUS English?

Do they feel a neglect to the other variants of the language?
I don't really understand this sentence / я не понимаю

Tell me, please, about differences of the tongues.
Regarding the question: I'm a native speaker of american english. From my own observations and interactions i'd say the differences between american english and the rest is:

a) pronounciation ("schedule")
b) vocabulary (vacation vs holiday)
c) colloquial sayings or slang (... i don't have an example)
d) sometimes spelling (color vs colour)

I've almost never had trouble understanding the other variants of english. I'd also say that we have variations of english just as wide here in the US (california, the south, midwest, northeast). Again, it's all pretty much understandable. This is so far the biggest difference between english and russian I've found: In russian the pronounciation seems to be very uniform which leads to few accents, whereas in English there are _lots_ of accents. Just in my own state (northeastern part), I could find you a half dozen people who speak with very different accents.

Jon