PHP, Ruby, Python, Java, or C++. All of them are nearly identical, so you can go from one to other fairly easily. After the basics the primary differences are names for functions and the syntactical layout of stuff. PHP is the one of the most widely used web-programming languages, and that knowledge can be fairly easily moved to another. C++ or Java would be a decent place to start too, as they're the most abundantly used languages for desktop-based applications and mobile applications. If you want to start right up with desktop/mobile apps my suggestion would be C++, that's what the BitCoinD/BitCoin-Qt applications are coded in.