have you tried?
Set Edge Chromium to en-AU via group policy
I have set Application Locale to en-AU, Set Spellcheck to en-AU and enabled Spellcheck.
If I check the Edge://policy/ these are all being applied at a device level.
The problem is, it doesn't install the en-AU language. Can someone please advise how we deploy that without doing it manually by clicking in Edge > Settings > Languages > Add > etc.