In Microsoft Cloud products the source of language configuration isn’t always consistent. Loop on the web uses the language preference from “My Account” or the Entra ID account.

It is the property preferredLanguage in the Entra ID account…,

…customizable via PowerShell and Microsoft Graph.
$Body = @"
{ "preferredLanguage": "de-CH" }
"@
Import-Module Microsoft.Graph.Authentication
Connect-MgGraph -Scopes User.ReadWrite
$Url = "https://graph.microsoft.com/v1.0/users/<UserPrincipalName>"
Invoke-MgGraphRequest -Method PATCH $Url -Body $Body -ContentType "application/json"
After making the adjustment, synchronization between products may take some time. Re-logging accelerates the process.

Ny experience is that Loop ignores my Microsoft Account (and browser) setting of English (Australia) and uses English (United States).
If I go into the Loop settings and change the language to English(Australia), within 24 hours it will revert to English (United States).
Hello Mike, I’ve seen the same behavior in my own setup. Loop ignores many country-specific language variants. For example, when I use de-CH, Loop on the web doesn’t respect it and switches to English. As soon as I set de-DE for my Entra account, Loop correctly stays in German.
I don’t have experience with an Australian language setting, but I think you have the same issue. The supported languages for Loop on the web are the display languages listed in My Account at https://myaccount.microsoft.com/settingsandprivacy/language. If the language variant is different, Loop will always use en-US.