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If you're using WordPress, you can write a quick plugin to do this. The following code amends the submit button's text value on the Registration Confirmation form:

add_action( 'civicrm_buildForm', 'my_prefix_alter_form_button', 10, 2 );

/**
 * Amend the Registration Confirmation form submit button value.
 *
 * @param string $formName The name of the form.
 * @param CRM_Core_Form $form The form object.
 */
function my_prefix_alter_form_button( $formName, &$form ) {

    // bail if not the form we want
    if ( $formName != 'CRM_Event_Form_Registration_Confirm' ) return;

    // drill down until we find the submit element and change its value
    foreach( $form->_elements as $element ) {
        if ( isset( $element->_name ) && $element->_name == 'buttons' ) {
            foreach( $element->_elements as $item ) {
                if ( isset( $item->_type ) && $item->_type == 'submit' ) {
                    if ( isset( $item->_attributes ) && $item->_attributes['name'] == '_qf_Confirm_next' ) {
                        $item->_attributes['value'] = __( 'Continue to pay via Paypal', 'my-plugin-slug' );
                    }
                }
            }
        }
    }

}

You can adapt this methodology to work with Drupal too. You'd have to create a module with an appropriately-named function, but the code inside the function should be identical.