HL bots are usually written in C++, however many are actually C-style C++ (i.e., use no classes)

bots for some other games (like Quake 2 or the official bot in Quake 3) are usually written in C.
If you want to do a HL bot, you can download the HPB bot source code at
http://planethalflife.com/botman and take a look at it.