modules/locale/views_handler_argument_locale_language.inc

<?php

class views_handler_argument_locale_language extends views_handler_argument {
  function construct() {
    parent::construct('language');
  }

  function summary_name($data) {
    return $this->locale_language($data->{$this->name_alias});
  }

  function title() {
    return $this->locale_language($this->argument);
  }

  function locale_language($langcode) {
    $languages = views_language_list();
    return isset($languages[$langcode]) ? $languages[$langcode] : t('Unknown language');
  }
}