Technically, anyone who has already downloaded the Developer Preview version of Windows Phone 8.1 can go ahead and set up their smartphone to activate its Cortana digital assistant right now. However, Microsoft is officially only offering Cortana to U.S. residents.
That situation may be changing in the very near future. A post today on the official Windows Phone UK Twitter page hints strongly that Cortana will expand to residents across the pond "soon"
Exactly how soon is still a mystery, but we would assume that UK users will be able to get a native version of Cortana up and running sometime before Windows Phone 8.1 becomes generally available. That's expected to happen during the month of June.
Microsoft has also confirmed that a version of Cortana made for the vast Chinese market will go live for Windows Phone 8.1 sometime before the end of 2014. However, other regions around the world will have to wait until 2015 before they will be able to access native versions of the digital assistant. There are also rumors that Cortana could made its debut in a future PC version of Windows.
Source: Windows Phone UK on Twitter