Add a payment method to an already existing Wordpress plugin

Hello, 1. I have the wp-invoice plugin installed on [login to view URL] which can generate invoices like this: [login to view URL] I want [login to view URL] as a new Payment Gateway added on the admin area of the plugin in : Wp-Invoice -> Settings -> Payment so I'll be able to use all payment options that Klarna offer. 2. I also need to connect the register form from this page [login to view URL] with wp-invoice plugin to be able to receive payments after the form was completed successfully depending on the language selected on the website to work for any language: ex: -if language is English, the user will complete the English form ([login to view URL]). -When the form was submitted successfully a new invoice will be generated according to what course he selected(WP-invoice -> Settings -> Line Items) with preferred payment method, he will receive the 'New Invoice' email found on Wp-Invoice -> Settings -> Email Templates (in English) -The website must be adapted to work on any language the website has for now knowing that other languages will be added using the TranslatePress Pro plugin already installed. 3. On the website the email address should be verified adding a new email address field that will verify the existing one on: [login to view URL]