In English it is Settings-General-Language and thanks to google translte look for Configuración general lengua.
This may help http://support.apple.com/kb/HT5255 to change anything else once in English.
Interesting thought. My son gave me his ATV2 when he bought the ATV3. I plugged it in and was able to login to his Netflix and Hulu accounts, as well as his iTunes account. He had failed to reset the ATV2 to factory defaults and all of his accounts had saved passwords. Not good if he had sold the thing on eBay instead of giving it to his old man.