MATT: elevated powershell
Get-WinUserLanguageList
# lists the languges available to the logged on user
$1 = New-WinUserLanguageList en-US
# create a new language list object that contains the language of your choice - e.g. es-ES for Spanish in Spain
Set-WinUserLanguageList $1
# replace the current language list with the one you've just created
Get-WinUserLanguageList
# lists the languges available to the logged on user - should show only the language you've set
Reboot