Add en-AU translation file

[leevigraham]

Description

Added an en-AU translation file. This en-AU translation file imports the en-GB translation file.

The purpose of this PR is to add English (Australia) to both Language and Formatting user preferences.

This is important becuase the src/i18n/Locale.php class uses the users locale when formatting numbers and currency (new Money Field).

Ideally the Formatting Locale drop down options would be independent of the language and include a more exhaustive list of locales.

Example OSX settings:

The same approach can be taken for en-CA.

[leevigraham]

ping @engram-design @lukeholder 🇦🇺🦘🥧

[engram-design]

FYI, we add 'defaultCpLanguage' => 'en_au' to our general.php config file, which allows this to show up in the user profile. But this probably couldn't hurt!

[leevigraham]

@engram-design In that case wouldn't the CP language still default to en-US given there's no language file for en-AU.

[leevigraham]

@engram-design A slightly better solution is to:

1. add 'extraAppLocales' => ['en-AU'], to config.general.php
2. add a new translation file in /translations/en-AU/app.php

<?php
# /translations/en-AU/app.php
return require CRAFT_VENDOR_PATH . '/craftcms/cms/src/translations/en-GB/app.php';

[brandonkelly]

Closing this in favor of #10519.

[engram-design]

@leevigraham Well, no idea how it works then! 🤷‍♂️