[Try this.](http://magento.stackexchange.com/questions/3259/make-magento-core-template-use-my-modules-translation-file/3284#3284)

To sum, specify an additional translation file under the core module's translation XPath and place your overriding CSV pairs in there.

    <frontend>
        <translate>
            <modules>
                <Mage_Sales>
                    <files>
                        <mr_storms_wicked_translations>Mage_Sales_Custom.csv</mr_storms_wicked_translations>
                    </files>
                </Mage_Sales>
            </modules>
    </frontend>