Hey there,

You can language override the following language constants which is at Backend > Extensions > Languages > Overrides:

COM_PP_CHECKOUT_STEPS_CONFIRMATION="Invoice Confirmation"

COM_PP_CHECKOUT_STEPS_PAYMENT="Payment"

COM_PP_ORDER_CONFIRMATION="Order Confirmation"

COM_PP_PRICE="Price"

COM_PP_CHECKOUT_DISCOUNT_CODE="Discount Code"

COM_PP_APPLY_BUTTON="Apply"

COM_PP_PAYMENT_METHOD_DEFAULT="The payment method for this invoice will be processed by <b>%1$s</b>"

COM_PP_CHECKOUT_CREATE_NEW_ACCOUNT="Create Your Account"

COM_PP_CHECKOUT_ALREADY_HAVE_ACCOUNT="Already have an account?"

COM_PP_CHECKOUT_LOGIN="Login"

You can check it out here too. https://docs.joomla.org/J3.x:Language_Overrides_in_Joomla

Can you have a try and see how it goes?