There is an error with the information on the turnitin webpage about where to place the language information
http://turnitin.com/static/knowledge_base/Installing_the_Moodle_Plugin.html

Moodle now expects the language options to be in the file /lang/<language>/assignment_turnitin.php and not in assignment.php as it used to be. I think this change occurred in Moodle 1.9.7

Preferably the file should be added in the Moodledata directory in the /lang/<language_local>/assignment_turnitin.php

This file should now look like this:
<?php
$string['typeturnitin'] = 'Turnitin';
$string['helpturnitin'] = '<Help comment>';
?>