Article How to remove 'Unknown Locale (qaa-Latn)' from fresh Windows Server installation? answers how to remove the qaa-Latn language from the UI once the user has logged on, that is, from the input indicator, etc.
However, on the Windows logon screen there's a also a choice of languages/keyboards and I haven't been able to remove it from there, not even after removing the language from
Computer\HKEY_USERS\S-1-5-18\Control Panel\International\User Profile - Languages
Computer\HKEY_USERS\S-1-5-18\Control Panel\International\User Profile\qaa-Latn