Currently that is not available in Twig directly. That lookup is in the class LanguageCodes.php, but it currently can only be used by PHP, ie in a plugin or something. You could do this easily enough in your theme or plugin class:
I'm using Antimatter theme here for example:
<?php
namespace Grav\Theme;
use Grav\Common\Language\LanguageCodes;
use Grav\Common\Theme;
class Antimatter extends Theme
{
public static function getSubscribedEvents()
{
return [
'onTwigPageVariables' => ['onTwigVariables', 0],
'onTwigSiteVariables' => ['onTwigVariables', 0]
];
}
public function onTwigVariables()
{
$this->grav['twig']->twig_vars['language_codes'] = new LanguageCodes;
}
}
Then in your page content (with Twig processing enabled) or Twig template you can use it like this:
{{ language_codes.getName('pt') }}