Most cell phones speak WAP. A small subset of them also understands Java and an another one (even smaller) understands other languages. Here is the which-one-does-what-by-whom...

Product Author Language(s) Desktop Version Mobile Version Has IDE License Comment UP.SDK OpenWave WML WMLScript HDML Freeware Comes with a simulator WAPtor WAPTop WML Commercial WML editor Mobile Internet Toolkit Nokia WML WMLScript Freeware Complete solution for WAP development Wireless IDE Motorola WML WMLScript Freeware Includes tools and features specific to Motorola platforms J2ME Sun Java Freeware Best way to develop cross-platform apps