Simplified Chinese doesn't support mods

@agathayin  It would be a good test to load the game with no mods at all and see whether any text is missing.  You don't have to mess with your existing game folder: just move the entire Sims 4 folder out of Documents\Electronic Arts and onto your desktop.  When you launch the game, a clean folder will spawn with no content, so you can have a look around without any mod influence.  And you can put your old folder back when you're done.

I don't know enough about modding to tell you which mods would require translation strings, and for which functions.  It's possible that for some creations, they simply need to add pointers to the new simplified Chinese text resources; others may require added translations on top of what the game offers.  At any rate, the modders themselves will know.