The sourcecode to the game DLL's that ship with DLL extendable games will most likely be written in C. So, most people who are just going to modify the existing code to make their patches are going to have to learn C. I'm going to be totally up front here - If you haven't done any C programming you're going to have a very tough time getting your foot in the door, and that is precisely why Inside3D exists. C is a powerfull, cryptic, hard-to-learn and often misused programming language (I like to think of C as "the programming community's latin"), and we are here to help you learn how to use it well to write game mods.
You can also write mods for DLL extended games (Quake2 etc...) with other languages like C++ and Delphi. These languages will offer alot of power to more advanced mods. Most of the tutorials here on Inside3D will be in C, though, because they will focus only on modifying certain aspects of gamex86.dll. There will most certainly, however, be tutorials with other languages such as C++ and Delphi.
Getting a compiler
You will need a compiler capable of generating a Win32 DLL in order to write Quake2 mods. There are a good many free C and C/C++ compilers available on the internet - check out the compiler lists for a list of known (to me) C and C/C++ compilers. Delphi is a proprietary language developed by Borland Intl., so you'll have to buy Delphi (currently version 2 or 3) from Borland or at your local software store.
Learning the language
For the most part, Inside3D will not contain any general C, C++, or Delphi tutorials aimed directly at teaching you the language. There are a couple of reasons for this: it would take alot of of time to produce a decent overview of any of these languages; I don't know how to teach someone these languages because I was never taught to program in one of these languages; and lastly there are already alot of good tutorials for learning them on the net. Check out the Learning page for a good list of comprehensive online C, C++, and/or Delphi tutorials I find. If you know of any good comprehensive tutorials on these languages, let me know.
DLL Central is the part of Inside3D that focuses on explaining how to write DLLs. The information here does not pertain to any specific game (Quake2, Unreal, etc...), but rather will help you understand how to write DLL's in general. DLL Central contains a couple sections that focus on specific topics:
Note: There is no list of Delphi compilers because Delphi is made by Borland specifically.