If I recall correctly, the systems Portugal, Spain and Italy already have were having talks to integrate with each other, in hopes of then being able to integrate with the new German/European initiative with minimal effort.
I've not looked it up again in a long while, but apparently we now have interoperability with MB Way and Bancomat. Idk how well it works though. https://bizum.com/es/internacional/
If you use it to pay in stored however, the issue is not really that you can't use MB Way in Spain, it's that Bizum is not that deployed in physical stores yet. And by yet I mean this shit started deployment like 5 years ago and no store I know near me takes it. Edit: never fucking mind! Apparently they are launching something similar to google pay in May this year.