The way I fixed mine is to got so settings -> Language
I had two settings in there, the first was English (United Kingdom) and the second was English (United States). If you press and holld the one that says English (United States), it pops up a menu which allows you to "Move up" (I.E. move the English (United States) to the top of the list, I guess so it gets priority over all the others.) It then asked me to restart my phone, and after it restarted, Cortana was there in my app list.
I was not on any sort of developer preview. I just updated my phone normally when it alerted me that there was an update available.
Hope this helps.