Just looked over the code of some planner we write for university. We decided to write in LISP because its called the AI language. LISP is a very old language. But with the newer standard Common LISP its fairly modern.
The great thing about it is that you can express your thoughts very quickly without have to look on details. The planner generates a plan (a list) out of a list of facts and a list of actions. LISP stands for LISt Processing, so its made for these things.
For further reading I recommend the LISP primer and Practical Common LISP.