[hrktb]

From the little I know US has 51 states with each their little twist on different subjects like taxation or company laws. EU has a set of global rules, no trade barriers between the countries, and financial transactions and payment also should belong to a unified space thanks to the SEPA.

Doing payment cross europe may have it's difficulties I don't know, but on the paper that's not so different from the US, and as a customer I don't feel friction dealing with other countries merchants.

For the language question, if you're building a payment processor, localization of your interface won't be your biggest problem. BtoB documentation and support is OK in english, customer facing interface should be localized, but even supporting the main 3 or 4 languages should let you access most of the market.