Apply Registration Code translation?

If you use the Promotions option for CB Subs, it displays a box to enter the code, as well as the tip; however the button to apply the coupon says:

Apply registration Code

Which is confusing for users (it's a coupon not a registration code).

I've looked all through the language.php file of the default language to find a string to translate, but I can't find one.

Can you tell me what string I would use to translate this button? (and not effect the wording of the button for applying the actual registration code)?

The default string for the button is "Redeem coupon": You can check this on this site.

Something must be overriding that language string on your site.