This will definitely work. Had the same problem on my mom's 1320 where on my 1520 Cortana worked fine. Do this.
Change region to US
Change region format to US
Add US English in languages and press and hold on it and you'll get the option to move it to the top and then you can press and hold and remove the second language below it.
Make sure your location is on and restart your phone and you will have cortana in your apps after the restart, then set her up and you're good to go.
If you follow these exact steps, it will definitely work. The language part is what people miss. You need to move it to the top and remove the other one.
Hope this helps everyone.