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It isn't. They aren't even different dialects. There are low frequency noun s and verbs that are even lower frequency in one area than another, but tha t doesn't make it a different language. I'm about the only poster here who uses "albeit" but that doesn't mean I'm speaking (or technically - writing) a different language.
The different dictionaries don't represent different languages, but differe nt markets. Noah Webster put a lot of misspellings into his dictionary of A merican English in the hope of squeezing out English publishers, and it di d work. The Australian Macquarie dictionary does include characteristic Aus tralian locutions (which you'd otherwise have to dig into the complete Oxfo rd - all 14 volumes of it - to find) but it's still a dictionary of the Eng lish language.
Marginally different. Australian English is less different from British Eng lish than it is from American English - when we watched "The Wire" we had t o use sub-titles to keep track of what was being said.
The "two countries divided by a common language" quote is usually attribute d to George Bernard Shaw, but nobody has found it in his published output. Wilde wrote a very similar line for a play in 1887, a few years earlier tha n Shaw is supposed to have said it.
If it were it would be written in the Greek alphabet, always a helpful clue . Russian is written using the Cyrillic alphabet, which has a few more charac ters.