Magento – Custom module: translate string in Mage::throwException

I'm new to Magento and I've just built a custom module for a store. The module observes the event sales_order_place_before, and checks with an outside source if an item is still in stock. When the customer wants to pay for the order and a product in the cart is out of stock, I throw an error in my Observer.php model. It appears as a popup for the customer.

if(!$success){
    Mage::throwException("item not in stock");
}

Is it possible to throw this error in multiple languages, for example English and Dutch? And how does that work? I can't seem to make sense of how to use translations in an observer instead of frontend. And since this is just a custom string, it could be anything, I would not be overwriting any Magento error.

EDIT:
My module's config xml (I've removed the actual namespace and module name). Do I define a helper like this? What do I put in the helper?

<config>
<modules>
    <Namespace_Modulename>
        <version>0.1.0</version>
    </Namespace_Modulename>
</modules>

<global>
    <helpers>
        <translations>
          <class>Namespace_Modulename_Helper</class>
        </translations>
    </helpers>
    <events>
        <sales_order_place_before>
            <observers>
                <Namespace_modulename_model_observer>
                    <type>singleton</type>
                    <class>Namespace_Modulename_Model_Observer</class>
                    <method>placingOrder</method>
                </Namespace_modulename_model_observer>
            </observers>
        </sales_order_place_before>
    </events>
</global>  

<frontend>
    <translate>
         <modules>
             <Namespace_Modulename>
                 <files>
                     <default>Namespace_Modulename.csv</default>
                 </files>
             </Namespace_Modulename>
         </modules>
     </translate>
</frontend>

I have created a helper with an empty class in Helper/Data.php in my module:

class Namespace_Modulename_Helper_Data extends Mage_Core_Helper_Abstract{}

I've added the csv file in app/locale/nl_NL