Regardless of the current culture applied, the date shown on top of a group of message uses InvariantCulture (English). In comparison the time break date is formatted according to the culture.

This comes from a bug in WPF. https://stackoverflow.com/questions/520115/stringformat-localization-issues-in-wpf/520334#520334

To work this around, override the Language property metadata of FrameworkElement.

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