Instead of the whole "do what you know", I'm going to try and answer this objectively. After all, some people are just looking to expand their knowledge -- nothing wrong with that
C++: Fast, efficient, but a bit of a steep learning curve for those who are new. Doesn't offer as much freedom as other platforms like the JVM, but is a good language for those who don't mind the extra work
Java: Semi-fast, not very efficient, but provides a good tradeoff with the amount of freedom the JVM gives you. Although the JVM eats up RAM like there's no tomorrow, there's no denying it's easy to prototype for and is quite nice for any skill level. You also get a lot of freedom with the language you choose, as Scala, Groovy and Clojure all fit under this umbrella.
Python: Although unofficial, the Python support for FRC is first-class. Testability, wonderful documentation and easy prototyping make it the language of choice for many. The syntax is easy to pick up for new programmers, but remains a powerful tool, although less speedy than the JVM and C++ alternatives.
LabView: Probably the most controversial language in FRC. Some love it, some hate it, and some don't even consider it a language. Although it's visual based, it runs quite well on the RoboRIO thanks to National Instruments. Doesn't give you the same degree of freedom as other languages, but a nice alternative for those who aren't quite ready to learn a full language, and who learn better visually.
Hopefully that list explains (almost) everything. If you want my personal, subjective opinion, I've taken a liking to using multiple languages. Java for control structure, Scala for dealing with data.