gistfile1.cs

        static App()
        {
            var lang = XmlLanguage.GetLanguage(CultureInfo.CurrentCulture.IetfLanguageTag);

            FrameworkElement.LanguageProperty.OverrideMetadata(
              typeof(FrameworkElement), new FrameworkPropertyMetadata(lang)
            );

            var ceTypes = new Type[] {
                typeof(System.Windows.Controls.DefinitionBase),
                typeof(System.Windows.Documents.FixedDocument),
                typeof(System.Windows.Documents.FixedDocumentSequence),
                typeof(System.Windows.Documents.FlowDocument),
                typeof(System.Windows.Documents.TableColumn),
                typeof(System.Windows.Documents.TextElement),
            };

            foreach(Type type in ceTypes)
                FrameworkContentElement.LanguageProperty.OverrideMetadata(
                    type, new FrameworkPropertyMetadata(lang)
                );
        }