SIMS4:Can't set in any other language

@CachalotKing  The U.K. version of Sims 4 is English-only, unlike the global version, which supports 18 languages.  (There's also an Eastern European version that only offers Russian, Polish, and Czech.)  But the packs should support the language of the base game.  Are you saying that your base game can be played in Chinese but Seasons is English-only?  Because that would be strange, and not in line with what we've heard about how the language lock works.

If you want to play in Chinese, you should first refund the English-only version you have now.  As long as it's been less than 24 hours since you launched the game, and less than 14 days since you bought it, you can get a refund automatically.

https://help.ea.com/en-us/help/account/returns-and-cancellations/

To get a global version, you can buy Sims 4 from Origin while you're in a country that supports the global version (and while Origin's storefront is set to that country); you can order a physical copy from one of those countries, because even the ones that come with a code and no disc should still support the global version; or you can buy a digital copy from an authorized third-party seller, but make sure to read the fine print and confirm you're getting the global version.  Here's a list of authorized sellers:

https://help.ea.com/en-us/help/faq/where-to-buy-ea-games-v2/

In any case, you'll likely need to have your current copy of the base game removed from your account.  Packs are supposed to be fine with any language the base game supports, again from what we've heard.