Using Symfony Translation component with Twig outside of Symfony.

composer.json

{
  "require": {
    "symfony/translation": "^5.0",
    "symfony/twig-bridge": "^5.0",
    "symfony/yaml" : "^5.0",
    "twig/twig": "^3.0"
  }
}

example.php

<?php

$loader = new \Twig_Loader_Filesystem('/path/to/templates/');
$twig = new \Twig_Environment($loader);

$translator = new Translator('en_GB', new \Symfony\Component\Translation\MessageSelector());
$translator->setFallbackLocale('fr_FR');

$yamlLoader = new \Symfony\Component\Translation\Loader\YamlFileLoader();
$translator->addLoader('yaml', $yamlLoader);

$translator->addResource('yaml', '/myapp/strings/fr_FR.yml', 'fr_FR');

$twig->addExtension(new \Symfony\Bridge\Twig\Extension\TranslationExtension($translator));