CMessageSource

public function getLanguage()
{
    return $this->_language===null ? Yii::app()->sourceLanguage : $this->_language;
}

abstract protected function loadMessages($category,$language);

public function onMissingTranslation($event)
{
    $this->raiseEvent('onMissingTranslation',$event);
}

public function setLanguage($language)
{
    $this->_language=CLocale::getCanonicalID($language);
}

public function translate($category,$message,$language=null)
{
    if($language===null)
        $language=Yii::app()->getLanguage();
    if($this->forceTranslation || $language!==$this->getLanguage())
        return $this->translateMessage($category,$message,$language);
    else
        return $message;
}

protected function translateMessage($category,$message,$language)
{
    $key=$language.'.'.$category;
    if(!isset($this->_messages[$key]))
        $this->_messages[$key]=$this->loadMessages($category,$language);
    if(isset($this->_messages[$key][$message]) && $this->_messages[$key][$message]!=='')
        return $this->_messages[$key][$message];
    elseif($this->hasEventHandler('onMissingTranslation'))
    {
        $event=new CMissingTranslationEvent($this,$category,$message,$language);
        $this->onMissingTranslation($event);
        return $event->message;
    }
    else
        return $message;
}