WPF RichTextBox and specific culture issues

Answer 1

I'm not sure if this will help, but it might be worth a shot.

In the Application.StartUp event add the following code

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

Or you can modify it to specify es-PE instead of the current culture.

This post might provide more in-depth details: Getting a WPF application to pick up the correct regional settings