While k is extremely terse, a lot of investment banks these days are using systems which utilise q and it's associated kdb+ database. q is basically a wrapper around k which makes it a lot easier to read and code in and which allows for sql-esque select statements from databases. This combination makes it far more flexible and useful for dealing with financial databases than straight k.

My rule of thumb these days is that the smaller and more obscure the hedge fund, the more likely it is that they're using an unusual set of languages. The more PhDs they have, the more likely they're using something weird like K!

Certainly the enterprise stuff at Spanish banks is run on Cobol, and there's still a fair amount of active Cobol development going on. (By enterprise I mean the bread and butter stuff, accounts etc.)

No idea about the trading / derivatives / whatnot, though.

And Haskell now, I believe.