To make a complete platform, you also need APIs, so there's more to it than just picking a language. You also need to figure out how to sandbox it; CINT appears to give programmers access to unrestricted pointers. You also want to get multiple browser vendors to agree, so some kind of specification is desired; CINT targets its own unstandardized subset of C. And you ideally want it to go fast, but CINT appears to be pretty slow:

http://benchmarksgame.alioth.debian.org/u32/compare.php?lang...

So there'd be some work to do. You could also compile the code, but complete C compilers are not fast, in browser terms.