How do I change the language of my novel to Australian (or British) English?
What do you mean by changing the language? Do you just mean having British or Australian spell checking and maybe grammar-checking — I never use grammar checking, so I don’t know if that makes any difference — or do you have more substantial changes in mind in terms of the way the language is used by Brits or Aussies, compared with Americans? If the latter, that’ll be down to you to rewrite, if the former you can set your Mac to use any of those …
… but which version of OS-X are you using? 10.6 has differences from 10.5 in spell-checking, etc. and presumably there were changes between 10.4 and 10.5 … I can’t remember … too long ago.
Mark
10.6: You’ll need to set it up in System Preferences under Language & Text, Text tab. You can select from Australian or British.
In 10.5 and .4, I don’t think there was a standard way to go about it, but you could change the system preference by loading up the spelling panel in any Cocoa application and choosing from the drop-down there.
I just wanted to add:
If you are just looking to change the language that is used for spelling and grammar checks, what you have to do is: right click somewhere in the document you want to edit; under “Spelling and Grammar” click on “Show Spelling and Grammar”. You will get a small HUD where at the bottom you will find a drop down list of available languages. Pick what you need. 
You probably will have to do the same for every single document I guess… or maybe using a “Edit Scrivenings” session might allow you to change a whole bunch at once, I am not sure.
(forgive MY spelling and grammar please, I’m not a native speaker 
)