code: public function getColumnNames(GetPropertyOptionsEvent $event)
      {
          if (($event->getEnvironment()->getDataDefinition()->getName() !== 'tl_metamodel_attribute')
              || ($event->getPropertyName() !== 'select_langcolumn')
          ) {
              return;
          }
  
          $result = $this->getColumnNamesFrom($event->getModel()->getProperty('select_table'));
  
          if (!empty($result)) {
              \asort($result);
              $event->setOptions($result);
          }
      }
docs: Retrieve all column names for the current selected table.
  
  @param GetPropertyOptionsEvent $event The event.
  
  @return void