Hi Jonathan,

I have list down the language constants involved.

COM_PP_THANK_YOU_FOR_PAYMENT="Thank You For Your Recent Purchase"

COM_PP_THANK_YOU_NOTE="We have sent your subscription confirmation details to your registered e-mail.<br />As soon as we receive the confirmation, the order status would be marked as completed."

COM_PP_PROCEED_TO_DASHBOARD_BUTTON="Proceed to Dashboard"

COM_PP_CHECKOUT_CREATE_NEW_ACCOUNT="Create Your Account"

COM_PP_CHECKOUT_ALREADY_HAVE_ACCOUNT="Already have an account?"

COM_PP_CHECKOUT_LOGIN="Login"

COM_PP_REGISTER_FOR_NEW_ACCOUNT_INFO="Before you are able to purchase, we require that you register an account with us first. Please click on the button below to sign-up for a new account"

COM_PP_REGISTER_FOR_A_NEW_ACCOUNT="Register for a New Account Today"

COM_PP_CANCEL_AND_RETURN="Cancel and return"

By the way, one method you can use to see the language constants is by emptying the contents in the following file then load the page again. Remember to restore back the content in the language file.
JoomlaFolder/language/en-GB/en-GB.com_payplans.ini