I created a new package/theme/skin from scratch in Magento 1.9.1.
The theme and skin load fine. I tried adding translations and cannot get them to load. All cache settings are disabled.
I've added the CSV file here:
/app/design/frontend/{my_package}/{my_theme}/locale/en_US/translate.csv
translate.csv:
"testing123","Translation Worked" header.phtml
<?= $this->__('testing123') ?> I added {my_theme}, which for me is named "responsive" to the admin at the :

The result is the header displaying 'testing123' instead of the translated 'Translation Worked'. What have I missed? How do I troubleshoot this?