Hello and welcome to SimV, Horse.
Echoing Crash's comments, it is not currently possible to change the accents in the default ATC system.
However, if I may suggest a very realistic alternative, it is the use of the VATSIM network. This is a virtual community of pilots and air traffic controllers that use realistic voice communications online in the Microsoft flight simulator series. With this type of experience, you are flying online with real people, in real time, speaking with you and giving you accurate instructions based on the latest real world procedures.
http://www.vatsim.net/Enjoy your stay at Simviation, we're a friendly group here, so don't be afraid to ask a lot of questions! ;)