Cortana will not be "officially" released for WP 8.1 it seems. However, it will be available for Windows 10 Mobile, and should be available, from my understanding, when it launches. I'm uncertain of it, but I know this new update in build 10586 for PC was supposed to bring Cortana to India, and I believe the new Windows 10 Mobile build 10586 was supposed to do the same. I've heard no official announcement at this time.