When customizing Magento emails, you have the choice of modifying the template through the Magento Admin Panel,
or through the system folder app/locale/xx_XX/template/email. The last option classifies as a core hack, and with every upgrade your changes will be gone. The first option requires you to modify code within a textarea and no syntax highlightning available.
This extension allows you to create file-based overrides of the Magento email templates within your own custom Magento theme. You can simply copy templates around, and modify the files using favorite IDE.
app/design/frontend/{PACKAGE}/{THEME}/locale/{LOCALE}/template/email
In the path, {PACKAGE} placeholder reflects your own theming package, {THEME} reflects the Magento theme you are
using and {LOCALE} reflects the language locale you are using - for example en_US or de_DE. Refer to the Magento documentation on how to configure theming and locales.
An additional feature of this module is that CSV translation files like Mage_Checkout.csv can also be overrridden in your theme. For instance:
app/design/frontend/default/mytheme/locale/en_US/Mage_Checkout.csv
This allows you to make any changes to any language file simply by copying files.