The problem is that /course isn't the parent directory of any of the current plugin types, so Moodle doesn't know to look for your lang strings there. For example, activity modules live in /mod, and course formats live in /course/format, so Moodle will look for /course/format/blah/lang/en/format_blah.php, but it wont look for /course/mergefiles/lang/en/mergefiles.php
The first thing you need to decide is what sort of plugin you're creating. I'd suggest that it could either be a Block or a Course Report, depending on how you want users to access it.
As an example, if you were creating a course report, it would have the Frankenstyle name coursereport_mergefiles. You'd move your code to moodle/course/report/mergefiles, and create a course/report/mergefiles/version.php to tell Moodle about your plugin. As detailed in the Course Report documentation linked above, you'll also want to create course/report/mergefiles/lib.php with the code required to add a link to your page in the course navigation.