I'm mainly an iOS developer, so I thought I might do some contribution, just in case.
So, here's my translation of the C# Unity client into Objective-C.
The Fossil delta algorithm is taken from the C++ library someone did for Cocos2D.
Websocket communication is done via the PSWebSockets library.
MessagePack decoding is done by the MPMessagePack library.
It's still a work in progress for sure, I haven't done a lot of testing on it, but so far, state synchronization works, and basic messaging seems to work for me.
Colyseus is a very useful tool, and I would like to see more interpretations of it in other languages, too.