It is being written purely in assembly language, with plans to let apps for it be written in assembly or C.
Programming in assembly language is interesting and fun, because you work very close to the actual hardware, dealing with registers, memory locations, addressing modes, bit manipulation, etc.
I've only done a little of it, much earlier, on the Commodore-64, the BBC Micro, and PCs running DOS, but enjoyed it.
I once bought a book that had the entire listing for a C-64 assembler, written in BASIC, using POKE, and entered it into the machine, including re-entering a large part of it due to finding bugs, probably due to typos while entering the hex codes. IIRC, it took me the better part of a week to complete that job and get a running assembler, but it worked :)