Avalonia/samples/Sandbox/MainWindow.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
        x:Class="Sandbox.MainWindow">
  <RichTextBlock 
    HorizontalAlignment="Center" 
    VerticalAlignment="Center" 
    Background="AliceBlue" 
    Padding="5" 
    IsTextSelectionEnabled="True" 
    FontSize="18" 
    FontWeight="Bold" 
    SelectionStart="24" 
    SelectionEnd="23">
    <LineBreak/>
    <Run>أَبْجَدِيَّة عَرَبِيَّة</Run>
  </RichTextBlock>
</Window>