I'd recommend c++ as well, just because there's so much you can learn about how the computer operates at a lower level than AHK exposes to you. The learning curve can be quite steep but it will make you a better AHK programmer. There are a lot of function prototypes and example code for the Windows api on MSDN written with c++. Learning this will greatly help when you go to use DllCall, access struct members, and manage memory (VarSetCapacity) in AHK.
A warning though, don't expect to write any big/interesting programs early on.
If you're looking for something easier, I rather like python and lua. They aren't terribly similar to AHK but they're very easy to get started with. Keep in mind these aren't developed for Windows specifically, so you'd need some third party libraries to do system specific things