Once you have installed WP8.1 on your device, you will simply have to change the region and language settings on your device:
Head over to the Settings menu -> Language and add English (United States) as a language as a new entry and make it the active language. You will need to restart your device once.
After that, change the region setting to “United States” in the region module of the standard Windows Phone Settings app. Head into Settings > Region and ensure “country/region” is set to the United States and “regional format” matches phone language. Changing the region setting will require a reboot.
After making the changes described above a Cortana app should be available in the standard app launcher.