The problem is you have added CUSTOM_FOLDER before /en/. I would follow this structure:
# Default files, keep them here.
/lang/en/validation.php
/lang/en/passwords.php
/lang/en/pagination.php
# Example for custom file
/lang/en/site/header.php
Example of namespacing: http://laravel.io/forum/03-01-2014-language-files-on-external-directory