Well, auto-generating cross-language bindings so that you can make function calls between code written in arbitrary languages would be a good start. Limited subsets of this exist, usually variations on "how to call C libraries from other languages" since that's the most common scenario.

For example, there's SWIG: http://www.swig.org/

Unfortunately, there tend to be subtle issues of impedance mismatch that make cross-languages bindings never feel quite right.