Depends on the type of task and the target platform — C#, C++ and Python to name a few.

And, of course, VHDL (in certain projects)...

99% chance it's C++, that's what essentially all trading firms use.

Yeah, all of the trading systems I've worked with have been in C++. At a previous employer, they had initially written their entire stack in Java. But, performance was an issue, as was cost (this was back Java still belonged to Sun and they were running Solaris on Sun hardware). Right before I joined, they had rewritten most of the java components in C++ and were in the process of moving off of Sun hardware & Solaris in favor of Intel & Linux. Front end stuff was mostly C++/MFC on Windows, later was C#/WPF. Perl & Python were both heavily used there, as well.