The English boards have, by far, the most activity, and have since the forum's creation. Some of this is driven by the US's economic might, and influence over the rest of the world, but I think a lot of it is due to Americans being the primary user's of the forum.
Theymos used to live in the US. I am not sure if he still does -- there are not many countries with more privacy protections, or free speech protections than the US, so I think he probably hasn't moved. He is the one who owns and is responsible for the forum. So it is reasonable to say the forum is based in the US.
Also, the
privacy page says the forum is subject to US jurisdiction, which is another argument in favor of saying it is based in the US.