As a workaround you can change the NumberNegativePattern to the default one like so:

System.Globalization.CultureInfo ci = (System.Globalization.CultureInfo)System.Threading.Thread.CurrentThread.CurrentCulture.Clone();

ci.NumberFormat.NumberNegativePattern = 1;

Thread.CurrentThread.CurrentCulture = ci;

Thread.CurrentThread.CurrentUICulture = ci;

Currently we're parsing Key.Decimal the same way we're parsing Key.OemPeriod. 
Our clients want to always type the culture decimal separator when pressing the Numpad separator key (del). 

Currently if the client uses a French culture, which decimal separator is comma - pressing the Numpad separator key does nothing.

Workaround: Handling the RadNumericUpDown.PreviewKeyDown method:
private void RadNumericUpDown_PreviewKeyDown(object sender, KeyEventArgs e)
{
	 var num = sender as RadNumericUpDown;
	 if (e.Key == Key.Decimal || e.Key == Key.OemComma || e.Key == Key.OemPeriod)
	 {
		 var textBox = e.OriginalSource as System.Windows.Controls.TextBox;
		 if (!textBox.Text.Contains(num.NumberDecimalSeparator))
		 {
			 textBox.Text += num.NumberDecimalSeparator;
			 textBox.CaretIndex = textBox.Text.Length;
		 }

		e.Handled = true;
	 }
} 

I am in need of a two digit display format using the NumericUpDown control. As I have to display the integer value "0" as "00" in all cases. This is a big draw back as I have to be complient to other systems.

-Stefan