[WordPress] Mini example plugin. Modify default (not translated) language strings of a plugin. Beware: Might result in performance issues when applied to many strings.

foo-custom-text-strings.php

<?php
/**
 * Plugin Name: Custom Text Strings for Foo Plugin
 * Description: Modify default (not translated) language strings of a plugin. Beware: Might result in performance issues when applied to many strings.
 * Version: 2014.10
 * Author: Caspar Hübinger
 * Author URI: http://glueckpress.com/
 * License: GNU General Public License v2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Modify default text strings of Foo plugin.
 * Assuming plugin intorduces "Foo_Plugin" class, has text domain "foo-plugin".
 *
 * @param mixed $translated
 * @param mixed $original
 * @param mixed $domain
 * @return string
 */
function gp1410201916( $translated, $original, $domain ) {

	/* Only strings from given plugin */
	if( ! class_exists( 'Foo_Plugin' ) || 'foo-plugin' !== $domain ) {

		return $translated;
	}

	/* 'Original string' => 'New string' */
	$strings = array( 'Wordpress' => 'WordPress' );

	if( isset( $strings[ $original ] ) ) {

		$translations = &get_translations_for_domain( $domain );
		$translated   = $translations->translate( $strings[ $original ] );
	}

	return $translated;
}
add_filter( 'gettext', 'gp1410201916', 10, 3 );