The Sims™ 4 is available in 18 languages, independent from the languages that Origin Client and Store support. This includes Simplified Chinese which was released in January!
If you wish to play the game in a different language than English, follow these steps:
In your Origin Client, once logged in, go to My Game Library.
On The Sims™ 4 Base Game, right click and choose Game Properties.
Click on Advanced Launch Options and choose the desired language from the drop-down menu.