Can I set is_rtl() to true?

You’ve got two options:

  1. In wp-config.php, set

    $text_direction = "rtl";
    
  2. If you’ve defined your own .po or .mo translation file for Pashto (is there really not one already?) there should be a string to translate

    • value: ltr
    • context: text direction

    Set the translation to be “rtl”. If that’s the user’s loaded locale then that will be picked up automatically.