The same things that make it hard to make one app for all of the EU also make it hard to make an app for the USA from outside the USA.

Even if you hire a UK or Éire citizen for their English skills, you have all the cultural reasons that means Tesco isn’t a supermarket brand in the USA and Walmart isn’t a supermarket brand in the UK even though both are English speaking nations. The language issues are also substantial, even though the cultural output of Hollywood has made it easier for many EU citizens to learn English than each other’s languages.