If you follow that reference through to the block class which is rendering the template, you end up at
public function getExcludeTaxLabel()
return $this->helper('tax')->__('Shipping Excl. Tax (%s)', $this->escapeHtml($this->getTotal()->getAddress()->getShippingDescription()));
$this->helper('tax')->__() tells you is that the
Mage_Tax module is responsible for translating the string "Shipping Excl. Tax (%s)". (This would be important if there were multiple modules with this string but you only wanted to translate it when rendered by a certain module.) You can find this string as part of the translation CSV files under
./app/locale/en_US/Mage_Tax.csv. Searching all other translation files shows that this string is unique in the translations list, so you can effect a translation via a custom
translate.csv file in your theme, or via inline translation in the admin. Everything you want to know about translations can be found here.