GBA ROM Programming Language

Question

Does anyone know what language ROMs (such as GBA ROMs) are coded in? I'd also like to know if there would be a simplistic way to decompile these ROMs.

Answer 1

GBA games can be programmed in any language, as long as it compiles to ARM assembly.

They're usually written in C++.

If you're interested in writing your homebrew games, see here.

Answer 2

I think you've misunderstood what ROMs are. They're not the actual executables (or what they would be on the original platform), rather they are image files that contains the executables. They can be written in many different languages.
See this wiki page for more info:
http://en.wikipedia.org/wiki/ROM_image