<TextBlock>
 <TextBlock.Text>
  <MultiBinding StringFormat="{x:Static res:MyResources.TestString}">
   <Binding Path="StringProperty"/>
  </MultiBinding>
 </TextBlock.Text>
</TextBlock>

<MenuItem
 Header="Stundenübersicht vollständig"
 Foreground="{StaticResource BasicForeground}"
 Command="{Binding OpenLink}">
 <MenuItem.CommandParameter>
  <MultiBinding 
   Converter="{StaticResource resourceStringFormatConverter}"
   ConverterParameter="{x:Static res:MyResources.TestString}">
    <Binding Path="StringProperty"/>
  </MultiBinding>
 </MenuItem.CommandParameter>
</MenuItem>

class ResourceStringFormatConverter : IMultiValueConverter
  {
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
      if (parameter is string)
      {
        if (values.Length > 0)
        {
          return String.Format((string)parameter, values);
        }
      }
      return null;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
      throw new NotImplementedException();
    }
  }