If you want a different than default language, you can go to the settings and select it under "Settings... General", then you should open the "Settings... Advanced" to define the folders you want to use for your downloaded game, your installers and also if you want to keep or delete the installers after using them (very useful if you want to install your games on a different drive).
Now, the easy part, you just scroll down on "My Games" until you find "The Sims 4", click on it, then on the small button with an "i" (for information), then click on Download.
To install the expansions (you can select as many or as little as you wish), just hover with the mouse over them and click on "Add to Downloads", repeating for each one you want in your game:
Wait until everything is downloaded and installed...
The small window on the right will show this when done:
Congratulations! Now the game is installed!! Each time you want to play you just need to run ZClient and open ZLOrigin after connected, then scroll down to the game, click on it, then Play!