<?php
<<<CONFIG
packages:
    - "twig/twig: ^2"
    - "twig/intl-extra: 3.x-dev"
    - "symfony/var-dumper: ^4"
CONFIG;

$loader = new \Twig\Loader\ArrayLoader([
    'ok_fr'    => "{{ 'fr'|language_name }}\n",
    'ok_fr_CA' => "{{ 'fr_CA'|language_name }}\n",
    'ko_fr_FR' => "{{ 'fr_FR'|language_name }}\n",
]);
$twig = new \Twig\Environment($loader);
$twig->addExtension(new \Twig\Extra\Intl\IntlExtension());

echo $twig->render('ok_fr'); // French
echo $twig->render('ok_fr_CA'); // Canadian French

try {
    echo $twig->render('ko_fr_FR');
} catch (\Exception $e) {
    dump($e);
}
/*
Twig\Error\RuntimeError^ {#159
  -lineno: 1
  -name: "ko_fr_FR"
  -rawMessage: "An exception has been thrown during the rendering of a template ("Couldn't read the indices [Names][fr_FR] for the locale "en_US" in "/path/vendor/symfony/intl/Resources/data/languages". The indices also couldn't be found for the fallback locale(s) "en", "root".")."
  -sourcePath: ""
  -sourceCode: ""
  #message: "An exception has been thrown during the rendering of a template ("Couldn't read the indices [Names][fr_FR] for the locale "en_US" in "/path/vendor/symfony/intl/Resources/data/languages". The indices also couldn't be found for the fallback locale(s) "en", "root".") in "ko_fr_FR" at line 1."
  #code: 0
  #file: "/path/vendor/twig/twig/src/Template.php"
  #line: 421
*/