WP apps can be written in HTLM5+javascript, C++, VB or C#. The last one is the most common choice.
There is also appstudio that supports TouchDevelop, which is apparently also a programming language, but I've never seen or used it.
TouchDevelop is weird. It limits what you can export to App Studio at the moment. I'd wait till full support for TouchDevelop code in App Studio or VS is given before trying it.
The most common is C#. The Absolute Beginners series covers it in great detail, and if I remember right (WC covered this in an article but dang if I remember when), Xamarin released an extension for VS that basically wraps your C# code in VS to work on iOS and Android.
But a5cent has it right with C# being the most common.