To get Cortana on your device you will have to play with few settings
? First of all, set region to US.
? Select your language as 'English(United States)' (if by chance you have any other languages in use, make sure that 'English(United States)' is on top of all.
? Then, go to settings go to 'Applications' part (Simply swipe right on settings screen) and select Cortana and make sure it is turned 'On'.
After all this you will get Cortana on your device