As there are 100s of crypto coins each with their own QT-client, how do exchanges manage the transactions? Do they use the core client of every altcoin in the servers or will they be programming code to do that? If coding is done, which platform or language is secure?
As there are 100s of crypto coins each with their own QT-client, how do exchanges manage >the transactions?
From what I have seen and what I understand, each exchange running on a website like cryptsy or mintpal is its own process. Each "exchange server" is a program running on the website's server. Because each coin's protocol necessitates its own wallet, they just have to have a lot of servers. The way lots of servers are banded together to handle a lot of requests is a complex science, and a lot of it is proprietary because businesses invest a lot in it.
Using a qt client in a server is rather inefficient, because there is not much need for a graphical interface. Using such a client is just unnecessary extra overhead. These websites can save a lot of money by optimizing server programs, which in turn allows for less server use, and less necessary investment in hardware.
Do they use the core client of every altcoin in the servers or will they be programming code to do that?
The most efficient solution for running a crypto transaction would be the barebones command line, as it has minimal overhead. It just takes requests and sends them through the network. This is easiest to do with just the core client, although all bitcoin clients have the core client in them. Next, sending through transactions would require a server side program, that would do the exchange math, and then send the proper funds to the proper wallets. This part would be custom coded. Peatio is not highly prevalent yet, and there isn't other "exchange software," so exchanges do write exchange software themselves.
If coding is done, which platform or language is secure?
Server side coding does not affect security because it stays on the side of the exchange. Server side security precautions like encrypting important data aren't really affected by platform. As long as the platform can encrypt connections and keep data safe, it's a secure platform. I realize that's kind of a blow off answer, but almost every server platform is made to be secure enough to run these types of things, and so are the coding languages.