Thanks for posting. If you're still setting up the device, you may reboot it to restart the setup. From the language option, you may now choose English as the language for your Chromecast with Google TV.
On the other hand, if the set up is already completed and you can understand the Spanish language, you may go to Settings > System > Language. Then, choose a language and select the radio button beside it. If not, please reset the Chromecast with Google TV and do the set up again to see the option for the language.