The Client Interface is for providing general information on a Client.
This is only available for clients, these won't work on a sever!

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.api.IClient;

The Client class is a globally registered Symbol, so you won't need to import anything, you can just use the keyword client

client.player returns the player running the client as an IPlayer Object.
client.language returns the language the client runs on as string.