Well, to start this i suggest u to read about routers and not CISCO. This will limit ur view. Read bout what routers and then go into CISCO. See how they work and how u configure it.
Then u should get the answer.
Basically, if u talk about routers, especially when u use routers such as Linux (linux box) then i will say it's C language.
Even the configuration & scripts are written are mapping to C's syntax.