Typically C++, Java or C#, although the technology in the currencies trading world has generally lagged behind say, US cash equities where exchanges are catering more to electronic customers (a substantial portion of the currency trading user base is still trading manually).<p>Reuters changed over to a more modern platform around 2013, but prior to that for electronic trading they would basically designate a Windows host for you that ran their GUI application plus a server, which you could connect to and would just forward your requests via COM to the UI.