I assume this is the problem:
Because there’s no “Australian English” translation effort in KDE, it doesn’t appear on this page. And due to a faulty set of assumptions on this System Settings page, being unable to choose it as your language using this UI results in LANG not being set to en_AU.UTF-8.
Can you make your ~/.config/plasma-localerc file look like this:
[Formats]
LANG=en_AU.UTF-8
If you make the contents of the file look like that and then restart the computer, I believe it should resolve the problem. But before we continue, can you confirm that it does? My expectation is that this solves the problem for everything outside of the login screen.
If this doesn’t work, try
[Formats]
LANG=en_AU
instead.