I found nothing in c/c++, found "bitwasp" in php, but only supports a handful of coins (I'm looking for the top ~20), and I'm having a hard time finding anything else. Is this why most applications usually support only Bitcoin? It seems like most of the currencies (except maybe IOTA and Ethereum) can be abstracted behind the same interface
DIrect BTC forks such as LTC, DOGE etc. should provide the same RPC calls, so worst case you could indeed just write a wrapper based on an existing library for at least those kind of alt coins.
However looking at the current alt coin market caps there are only few direct BTC forks left in the top 20 (well, LTC and BCH to be exact, but maybe I'm missing some).