There is no need to do this sort of project in C. Personally, I recommend every programmer learn a bit of C. (It's a much smaller, simpler language than C-Sharp, so it's actually a lot easier than you might think!)
That said, I write GUI code for Linux most days at my day job, and the main language I work in is Python. Many people find Python a very convenient language for writing GUI stuff like you describe.
You will want to learn a bit about the esoterica of what X runs when it starts. It's actually pretty straightforward to boot to a single app. There are various ways of doing it, depending on whether you want somebody to have to log on first, or have some way to get to a "normal" desktop for management. But, once you decide which config file you need to add your application to, it doesn't matter what language that application is written in. C, Python, bf, per, C++. X just knows it is running "some program." There are bindings for popular (and less popular) GUI API's for most languages you would care to write in.
That said, C-Sharp isn't especially well supported on anything but Windows, so I woud suggest not trying to do it in that. There is some Linux support, but everybody I know who has used, has found the complexities and limitations to be a bit maddening. (I've never actually sued C-Sharp in anger, so I can't speak about it first-hand.) It is a language invented by Microsoft to run on their own platforms, so it isn't very surprising that it would be most at-home on Windows.
I would suggest looking up PyQt/PySide. Writing Python code which uses the Qt UI framework is easy, and very interactive. (You can build UI code "live" on the python command line interface, which isn't possible in C. This makes it very easy to figure out what you want to do. You can set properties on widgets in a window while it is open, and play with it while you tweak it.) The new "declarative" UI stuff with QML is designed for things like pretty cell phone UIs, so it should be easy to make something that works as a themed game loading menu that doesn't look like one of my boring business apps.