Is there a way to change what the player types in the chat? For example, if the player types "Hello world", the chat shows "<Player> Nice to meet you" instead after the player pressed enter. I'm planning to make this mod client sided too.

You will have to do some serious hackery to achieve this.

First of all, understand the structure of Minecraft's netty pipeline (if you don't know how netty works, learn it). It is set up in NetworkManager.createNetworkManagerAndConnect / provideLocalClient. You need to create an OutboundChannelHandler and insert it into the pipeline before Minecraft's encoders. That way you can intercept any outbound packets, in this case you need to listen for C01PacketChatMessage.

 

Do not do this if you don't understand netty!

Oh wow, that is surprisingly difficult(However, I will attempt to try it.). Thanks for telling me that!