Setlanguage As Default

Hi,

I would like to have the German language for my website.
I tried this but e.g. login error messages like "Username cannot be blank." are still in English.
Can you please help me?

index.php

<?php

// change the following paths if necessary
$yii=dirname(__FILE__).'/../yii/framework/yii.php';
$config=dirname(__FILE__).'/protected/config/main.php';

// remove the following lines when in production mode
defined('YII_DEBUG') or define('YII_DEBUG',true);
// specify how many levels of call stack should be shown in each log message
defined('YII_TRACE_LEVEL') or define('YII_TRACE_LEVEL',3);

require_once($yii);
Yii::createWebApplication($config)->run();
Yii::app()->setLanguage("de");