Create a file /locales/en_US/_global.php and add your "translation" to selected messages. Add only the strings that you want to change. The phrase must completely match the original, you can find exact strings in /locales/fr_FR/_global.php

<?php
/*
 * Global lang file
 */
if( !defined('EVO_MAIN_INIT') ) die( 'Please, do not access this page directly.' );

$trans['en_US'] = array(
'__meta__' => array('format_version'=>1, 'charset'=>'iso-8859-1'),
'Original text' => 'My new text',
'Categories' => 'Chapters',
'Blog' => 'Book',
);
?>