How to make all text upper case / capital?

You still can use a converter, just set the textvalue in the source of the binding :

<TextBlock Text="{Binding Source={StaticResource String1},  Converter ={StaticResource myConverter}}"/>

I think this will work for you

<TextBlock Text='{StaticResource String1}' Typography.Capitals="AllPetiteCaps"/>

For font capitals enumerations https://msdn.microsoft.com/en-us/library/system.windows.fontcapitals(v=vs.110).aspx

To complete Peter's answer (my edit has been rejected), you can use a converter like this:

C#:

public class CaseConverter : IValueConverter
{    
    public CharacterCasing Case { get; set; }

    public CaseConverter()
    {
        Case = CharacterCasing.Upper;
    }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var str = value as string;
        if (str != null)
        {
            switch (Case)
            {
                case CharacterCasing.Lower:
                    return str.ToLower();
                case CharacterCasing.Normal:
                    return str;
                case CharacterCasing.Upper:
                    return str.ToUpper();
                default:
                    return str;
            }
        }
        return string.Empty;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

XAML:

<TextBlock Text="{Binding Source={StaticResource String1}, Converter ={StaticResource myCaseConverter}}"/>

Rather than using a converter, you can use the tag CharacterCasing in a TextBox but in your case, it doesn't work on a TextBlock.

<TextBox CharacterCasing="Upper" Text="{StaticResource String1}" />