Yeah, unfortunately the only language that will work well with OP2 to produce mission DLL's is Microsoft Visual C++. We use version 6 but I'd imagine versions 5, and 2005/.NET (unmanaged, so no C#) would work fine.
We eventually plan on writing an interface where all OP2 functions would be exported thru COM, so almost any language that supports COM could be used to write OP2 DLLs.
Tools have been written with both VC++ and Visual Basic, there's no set language that we agree to use for writing tools, so any language that you want is fine! VC++ is the only real restriction, for writing OP2 missions.