I believe Valve already supports all the local payment systems in Europe, though they’ll only show it to you if you’re in one of those countries (the payment processing flow asks you which country are you paying from upfront and then uses that to display the various payment systems available for that country).

Same for GOG, by the way.

The problem is mainly that to sell to anywhere in Europe a seller has to integrate with the many local payment systems out there, which is a lot of work for a small seller.