Fix PHP Notice in Translator class

`Translator::getTranslatedMessage()` causes a PHP Notice due to a change made in https://github.com/zendframework/zf2/commit/7422c37dd8c24dc892ea0a2aa92054c60f18a794#L0R398

    Notice: Undefined index: en_US in .../Zend/I18n/Translator/Translator.php on line 402

This change adds an `isset()` to fix this, so that the index is checked as it used to be.