I would actually guess that C is the most common language for N900 apps. GTK+ is C and most of the N900 apps are using GTK still.

However, as i said in my previous post, the future (for maemo and in general) is Qt, which means C++/object oriented programming or one of the application layers.

Still recommend you learn the principles of object oriented programming, start with python as a language, modify a simple existing program until you are familiar enough to make your own, and then worry about C++ when you want to optimise that program. Python easily binds to C++, making the transition or mixing of the two fairly straightforward.