[tmaly]

my previous comment "mostly C++" got down voted. I happen to work at a firm in this industry. C++ is the language used most in HFT.

Aside from this, you can verify this by just looking at the job postings.

[chollida1]

Not sure why you were downvoted, but your answer had more than a few issues.

1) It was added well after I had already added an answer that included C++

2) it was only 2 words with additioanl information meaning that it didn't really add any value to the question.

I have no doubt you probably know the HFT scene and could probably add some color to people outside of it, but your first answer didn't really help in any appreciable way and this was probably why it was down-voted.