An example of app/config/config.php source: http://web-technos.blogspot.com/2013/04/internationalization-with-fuelphp.html

config.php

<?php
  //source: http://web-technos.blogspot.com/2013/04/internationalization-with-fuelphp.html

  return array(
    'language'           => 'en', // Default language
    'language_fallback'  => 'en', // Fallback language when file isn't available for default language
    'locale'             => 'en_US', // PHP set_locale() setting, null to not set
    'locales'            => array(
      'en' => 'en_US',
      'fr' => 'fr_FR'
    ),
  );
?>