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

Importing the package

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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;

Where to find the client class?

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

What to do with it?

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