The reason is straight forward. They have data that shows most people outside the US have the setting wrong. For the .com (American!) domain the default is English. For each country specific site the default is the primary local language. You can change the default via a cookie.
If only. The geolocation talked about here is taking place on the .com domain. Worse, even if you manage to switch it to english (they have been hiding that link more and more) it will still use geolocation to push search results in the "local" language.