How can I set the substitution property by overriding the default substitution ?

In the following example, the first TextBlock does not have the Substitution property set, so the algorithm displays Arabic digits as expected. However in the second TextBlock, the substitution is set to ’European’, overriding the default substitution for Arabic numbers and hence European digits are displayed.

[XAML]

<Page x:Class='WindowsApplication.Window1'
    xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
    Title='Code Sample' Height='300' Width='300'
>
    <StackPanel>
      <TextBlock Language='ar-SA' 
         FlowDirection='RightToLeft'>????: 1+2=3
      </TextBlock>
      <TextBlock Language='ar-SA' 
         FlowDirection='RightToLeft' 
         NumberSubstitution.Substitution='European'>????: 1+2=3 
      </TextBlock>
    </StackPanel>
</Page>