This is my binding (shortened, Command-Property is also bound)
<MenuItem Header="Key" CommandParameter="{Binding StringFormat='Key: {0}', Path=PlacementTarget.Tag, RelativeSource={RelativeSource AncestorType=ContextMenu}}"/>
The Tag-Property of ContectMenu's PlacementTarget is a String like
"Short.Plural"
What i expect to receive in the Command-Handler is:
Key: Short.Plural
But what i acutally receive is:
Short.Plural
Label does not use StringFormat but ContentStringFormat. Use it this way:
<TextBlock x:Name="textBlock" Text="Base Text"/>
<Label Content="{Binding Path=Text, ElementName=textBlock}" ContentStringFormat="FORMATTED {0}"/>