WPF formats non-string objects to a string using a fixed/hardcoded culture (en-US) regardless of the current culture (as in Thread.CurrentThread.CurrentCulture / CultureInfo.CurrentCulture).

Suppose you have a simple DataContext (ViewModel) like so :

public class Data
{
    public DateTime Now 
    { 
        get 
        { 
             return DateTime.Now; 
        } 
    }

    public string NowText 
    { 
        get 
        { 
              return DateTime.Now.ToString(); 
        } 
    }
}

Binding an instance of `Data` to a view, with two TextBlocks each of which using ‘Now’ and ‘NowText’ respectively will yield different results if your CurrentCulture is NOT en-US.

Working on an internationalizable WPF app I’ve found out this sad truth. I don’t fully understand why this happens but I’ve found a way to “fix it”.

Somewhere before any piece of UI is shown on the screen (for example in App.xaml.cs in ApplicationStartup) just place this little piece of code :

FrameworkElement.LanguageProperty.OverrideMetadata(
  typeof(FrameworkElement),
  new FrameworkPropertyMetadata(XmlLanguage.GetLanguage(CultureInfo.CurrentCulture.IetfLanguageTag)));

It sets for any FrameworkElement (TextBlock and Label are included) the formatting culture to the current culture. Of course you might need to change it again if the current culture changes (for example an on-the-fly UI language changes).