The EPI is not a German governing body. It is a partnership of banks who decided to implement Wero. All of them had to work to support it in addition to their existing systems, though it is not exceptionally hard because it's just a frontend for SEPA. If Spanish banks want to provide Wero to their customers then they'll join the EPI and have the same say as everyone else. But again, it's just fucking QR codes that translate to "make an instant SEPA mandate to send X โฌ to Y IBAN" so I don't understand what there is to be scared of on a technical level.
I don't understand the point of fragmenting the Eurozone for the sake of fragmenting the Eurozone. Integration is in the long run more complex, expensive, and less user-friendly than standardization. What is so good about competing solutions that they should not eventually be replaced by a common digital payment system?