This plugin enables to see the source and the target of each translation word/phrase/sentence. When you use other than English locale, all messages that are translated via gettext will be shown in both the source language and the target language.

For example, assume your using a French vesion of WordPress. The word “error” is “erreur” in French, so when you encouter a error, you will see “Il y a une erreur …” Using this plugin, you will see “There is a error …” and “Il y a une erreur …” at the same time, it will be useful when you are reviewing translations.

add_filter('gettext','mytranslate',20,3);
function mytranslate($translated_text,$text,$domain) {
    return $text . $translated_text ;
}

Here is a screenshot for wp-login.php. In this picture, English is the source, and Japanese is the target.