Thanks very much for your reply.

The /includes/languages/YOUR_LANGUAGE_NAME_HERE/modules/order_total/ot_subtotal.php file is there and looks ok.

Another data point:
Since I was struggling to isolate the problem. I decided it would be quicker to install a new instance of Zen Cart in a test account. After installing the new instance, I imported the production database into the test database associated with my new Zen Cart instance. When I displayed the invoice the "MODULE_ORDER_TOTAL_SUBTOTAL_TITLE" was displayed rather than the correct value of "Sub-Total". This is that same behavior exhibited by my production instance.

So it looks like a database problem rather than a coding or configuration issue.
If you agree, where should I start looking?

Thanks for all you do for the Zen Cart community.