As far as I know you'll need to know some of the C langs. Such as C or C++. Not sure if the Original R4i Firmwares are written in either of those, but most likely C or C++. If anything, you can make one using DevkitproARM using C or C++. That and there are tons of libs out there for your needs, so go ham.
But you'll need to learn those langs in order to basically do anything; and learn how DevkitARM and the ARM (7 & 9) Arch works.