Random numbers are not that meaningful but the US has 300 million habitants, the EU 500 million. Comparing GDP the EU states grouped together are a bit less than a tenth bigger than the US.
I think there can plenty of reasons to start a business in the US first (looser regulations and local talent are what comesto my first to mind) but would you lose business starting in a bigger market ?
The EU has more people, but they're spread through 28 countries, each of which probably has different laws regulating financial transactions and payment processing, making it a lot more work.
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.
I think there can plenty of reasons to start a business in the US first (looser regulations and local talent are what comesto my first to mind) but would you lose business starting in a bigger market ?