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Observer doesn't save order
public function addLangToOrder($observer){
        Mage::log('catching convert_quote_to_order_after');
        $order = $observer->getEvent()->getOrder();
        $order->setCustomerLanguage(Mage::app()->getStore()->getCode());
        $order->save();
        Mage::log($order->getCustomerLanguage());
    }

<events>
    <sales_convert_quote_to_order>
        <observers>
            <accustomer>
                <type>singleton</type>
                <class>Ac_Customer_Model_Observer</class>
                <method>addLangToOrder</method>
            </accustomer>
        </observers>
    </sales_convert_quote_to_order>
</events>

$customer_lang = 'customer_language';
$installer->addAttribute('order', $customer_lang, array('type'=>'varchar'));

<config>
    <!-- : -->
    <global>
        <fieldsets>
            <sales_convert_quote>
                <customer_language><to_order>*</to_order></customer_language>
            </sales_convert_quote>
        </fieldsets>
    </global>
    <!-- : -->
</config>