Are any other apps in Spanish? Such as Play music? Might be a setting with the Google account and not the device.
I have a US account, associated with a US vredit card. and a Nexus 7 purchased in the US. All my Google and N7 account settings specify the language as English. At the moment I am in Mexico. Google Play is in Spanish and all prices are in Mexican pesos on my N7 here. Even using a Windows computer and going to the Google Play website I have great difficulty getting English, and when I do find English the prices are still in Mexican pesos.
However, on my N7 Google search still gives me US results, while on my computer the website www.google.com
cannot be reached from here (instead I am redirected to www.google.com.mx
and get results pertinent to Mexico -- kinda silly if I am looking up a hotel in Harrisburg PA)