I'm Europe-based, I feel your pain. Every time I read about some new potentially awesome payment service, but only for the US-audience it consistently manages to make me grumpy all day thinking about all the lost business possibilities.
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 ?
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.