C# – WPF XAML StringFormat DateTime: Output in wrong culture

Label.Content is object so you can't use Binding.StringFormat there as the binding's target type must be string in order for it to work.

Two work arounds are: use TextBlock instead of Label and bind the Text property.

Use Label.ContentStringFormat i.e.

<Label ContentStringFormat="Current Value {0} of 10" Content={Binding ...} />

You only need to escape the string with {} if your first character is a {

I'm going to chime in here.

I successfully did this using the OverrideMetadata() method that the OP mentioned:

var lang = System.Windows.Markup.XmlLanguage.GetLanguage(MyCultureInfo.IetfLanguageTag);
FrameworkElement.LanguageProperty.OverrideMetadata(
  typeof(FrameworkElement), 
  new FrameworkPropertyMetadata(lang)
);

But, I still found instances in my WPF in which the system culture was being applied for dates and number values. It turned out these were values in <Run> elements. It was happening because the System.Windows.Documents.Run class does not inherit from System.Windows.FrameworkElement, and so the overriding of metadata on FrameworkElement obviously had no effect.

System.Windows.Documents.Run inherits its Language property from System.Windows.FrameworkContentElement instead.

And so the obvious solution was to override the metadata on FrameworkContentElement in the same way. Alas, doing do threw an exception (PropertyMetadata is already registered for type System.Windows.FrameworkContentElement), and so I had to do it on the next descendant ancestor of Run instead, System.Windows.Documents.TextElement:

FrameworkContentElement.LanguageProperty.OverrideMetadata(
  typeof(System.Windows.Documents.TextElement), 
  new FrameworkPropertyMetadata(lang)
);

And that sorted out all my issues.

There are a few more sub-classes of FrameworkContentElement (listed here) which for completeness should have their metadata overridden as well.