There's not really much work to speak of yet. Last I looked at i2p, it carried a JVM with it and the C impl was not full featured iirc. Will have to look again.
The C++ routers have now largely become feature complete, IIRC they've even ported I2P-Bote to work on them.
And you have the JVM reference router still being developed, so there's at least three different implementations of the router and that helps with finding bugs and the like.