PowerShell sessions don't support mixed-culture values such as 'en-DE'
Note: This may ultimately require resolution in the .NET framework.
Recent versions of Windows 10 support mixed culture values such as `en-DE` (sic) (since version 1703 - see https://msdn.microsoft.com/en-us/library/cc233982.aspx)
After using, e.g., `Set-Culture en-DE` to set the culture for future PowerShell sessions (which in itself succeeds, as you can verify at registry location [HKEY_CURRENT_USER\Control Panel\International]), subsequent PowerShell sessions quietly fall back onto culture 'en-US', as reflected in $PSCulture and [cultureinfo]::CurrentCulture.
Note that such a mixed culture *is* honored in PowerShell *Core*.
Michael Klement commented
Small correction: culture 'en-DE' is available since W10 version 1607, not 1703.