Add zh_cn.json

[Rain-Flying]

Thank for your guiding me!
I have checked the lang file. It is completed and runnable in real game, which tested in Resource Pack.
Safe to merge, right?

[dphaldes]

Thanks for the PR! While we are not sure whether to include this directly in the base mod since we cannot verify ourselves, but we are considering a language resource pack that will be shipped along with the mod that players can enable manually. We would like to include the contents of the PR in that (if you are fine that).
So hold on till we set up for that. Just letting you know that we have seen your PR :)

[Rain-Flying]

Thank you~ You can surely pack the file into an optional Resource Pack like the Vanilla Programmer's Art!
And don't worry to merge it, it has been proofread by some experienced Chinese BM3 Players. You can see those updates of the original file. They're all suggested revises.
If you have any questions about my translation, feel free to ask me!