The official client is coded in C++ using Direct3d for the graphics. The MapleRoyals source code and scripts use a combination of Java, Lua and Rust.
I think MapleRoyals use English, occasionally Spanish and I'm sure some addition of European languages. 100% sure there's Taiwan No.87 though.
Not yet For Mac compatibility we have to use Wine which is a program that creates a Windows skin for applications to let them run on Mac. To my knowledge, the newer version is quite troublesome to get working with Wine on Mac (I tried for a while). But there was a new Wine version released a few days ago, haven't played around with it yet to know if it'll make a difference. But to answer your question, I don't think you would (or should) need to write any Objective C to make it work.
Full disclosure, I've been working with the latest version of Wine myself when I saw that it came out and I've gotten no further than I did when experimenting with Wine v1. I was having issues installing and configuring DirectX and I think that is where it is ultimately failing. Some dll is not properly loading or doesn't exist that needs to be there. I just simply don't know enough. One day, one day
Is there any chance that I can understand those super-hard-Java code from a C++ code?, cause I looked at something like OrdinMS source code and its sooooo hard to understand how everything works
Wasn't odin's source a complete mess anyways... thats atleast wat i recall (been many years tho might have improved?)
It should be one of the easiest language to pick up nowadays, which is also high in demand. If you know C++, it would help. Because Java is very much like it, minus things like memory management. You shouldn't refer to Odin anymore, and like what @Rainheart said, it was a complete mess, but not surprising. Take a look at other active-development open source packages in RZ instead.