mm...
it can be done in vbscript and tried with wmi but as I'm not wmi-quru, couldn't make it happen.

gave up at:
code:
$objWMIService = GetObject("winmgmts:\\.")
$colOperatingSystems = $objWMIService.InstancesOf("Win32_OperatingSystem")

For Each $objOperatingSystem In $colOperatingSystems
? "-Locale:          "  $objOperatingSystem.Locale
$objOperatingSystem.Locale = "0409"
$objOperatingSystem.put
? "-Locale:          "  $objOperatingSystem.Locale
Next
get $
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