Technically, any of it could be, if you write a compiler to do so. OSes have been done in Java (JNode), .NET (MOSA, Singularity, SharpOS, Cosmos), Haskell (HOUSE), Python (Unununium), etc.
Edit: I see a lot of people talking about the very lowest level being an area where this couldn't be done; this isn't true.
There's no reason that the compiler for X language can't be extended to handle any low-level operation and expose it to the language. All functionality can be achieved from any language, it's simply a matter of picking the right tool for the job. Sometimes this is Python, sometimes this is C, sometimes this is assembly.
Look to projects like Cosmos and SharpOS to see a pure high-level OS Done Right (TM).
