Originally Posted by pludi

For a pen tester knowledge of programming languages is rather a minor point, as they are used mostly to glue together various tasks or automate others. More important is a good knowledge of networking protocols (IP, TCP, UDP, HTTP, SMTP, ...), how instructions are executed (regardless of the language it's written in), how process memory is organized (stack, heap, ...), and how to use and combine the tools already available (nmap, netcat, tcpdump, metaspl0it, various intercepting proxies, ...)