Short answer:
Python – for fast development
Java, C/C++ – for speed
Some sources for further study:
Which language should I use for Artificial intelligence (Java, Python)
Good Programming Languages for AI (Lisp, Python)
Should I learn Lisp, Python, or Java to learn artificial intelligence programming techniques? (Lisp, Python, Java)
Why do most programmers prefer c++ over java or c# for artificial intelligence/machine learning? (Matlab, Python, JAVA, C/C++)
What is the best programming language for machine learning? (Matlab, R, Python, JAVA, C/C++)
http://blog.samibadawi.com/2013/02/scala-vs-haskell-vs-python.html (Scala, Haskell, Python)
http://programmers.stackexchange.com/questions/167701/recent-programming-language-for-ai (Lisp)
Google AI Challenge: Languages Used by the Best Programmers (Top 3: Java; C++; Python)
Misc:
Nice article to compare Python v. Java: http://www.programcreek.com/2012/04/java-vs-python-why-python-can-be-more-productive/