I saw on the onekey website that simplified Chinese has been translated
But why is it displayed in English when I use the game?

As you have said, many language translations are fully approved and complete. Translation support was added a while back on the old development branch, and if you were to place the zh_cn.json file from OneSky inside those builds it would display in Simplified Chinese.

Translation support is something that has yet to be added to the newer releases of Sodium. All I can do is to refer you back to the original issue, and ask for patience