My company has a large C# code base. Well over half of this code is our core engine for creating, reading, modifying, calculating
If i was doing something like cross platform cross language support, I would create a 'common api' since the languages are similar in syntax you could make a translator easy enough. Then instead of calling java or .net apis directly from the core, you would call your 'common api' which would re implement the java and .net apis you would need. In this way you could create a cross language sandbox if you will. Since the main differences in java and c# are object definitions, I would get those by reflecting the C# dlls, and then reconstruct the constructs, then it would be easy to have an interpreter run through and implement the function bodies and convert properties to getters setters already knowing the structure of the files. This of course is assuming .net 2.0, some of the features in 3.0 and 3.5 become very difficult to 'interpret'
It would be complex, but probably not as complex as reconstructing a core in java by hand, and having to have 2 teams working on them seperatly. If this idea sparks some inspiration i might create one. I would really rather see a simpler stable mono installation for mac.
Basically I think a code level interpreter based on a set of common api classes is something very possible to write with a team in a week or two.