I want to enable Cortana outside of the US, but I can't change my phone language to US English. Does anybody have an idea on how to fix this?
Cortana is technically only available in the United States during its initial beta period. However, if you change your region settings to United States, you should be able to access this feature. Microsoft tells me that doing so could impact Windows Phone Store access, but I've not heard of this actually happening to anyone.
Changing the region
Windows Phone Store access
If you must use Cortana at this point and find that your App Store isn't working as it normally does, you can still install the apps you need via the WP website.