I just managed to make a new language - UK English is close enough, except that County should be State and VAT Registration Number would be better as ABN.
First I tried copying en-US.xml and cutting and pasting the relevant bits from definitions.xml - which should have been alright, except I think the field: 'introduced="6.0.0" ' was fouling it up so that the Languages page in the admin interface wouldn't load.
Then Deleted that attempt and saw the Create Language tab in the Languages admin interface - which was really great, but the xml generated the tags <locale>en-AU</locale> which should have been <code>en-AN</code>. It mostly seemed to work, but the links to edit, download and delete the language were truncated so only served to reload the languages page. ALso the flag didn't work - I had uploaded a flag png through cpanel.
But after changing "locale" to "code" in my cpanel's text editor it works better.
Still haven't got a successful test run to see if ABN replaces VAT REgistration Number on the generated invoices, but feeling a lot closer.