You should be able to easily find a freelancer by asking any Chinese people. Just look for someone who speaks very good English. Incompetent translators usually don't understand what they're translating.
Also, I suggest you to spend time communicating with your translator, instead of simply providing an excel. Translators need to know the context of the material they're translating. If they're translating a game, show them the game, if they're translating the app, show them the app and explain to them what it does. Or they will make hilarious mistakes. For example, "The Elder Scroll" was once translated to Chinese as "Old Man Scrollbar", "Lord of the Ring" as "Lord of Horse Racing Track".
Also would be good if he could (or has a friend who can) be my community manager for China since I don't want Chinese users to feel left out just because I can't speak their language.
You can send me an email (see my profile) about the details.
As for your question, there are some upwork type websites but I am not sure of the quality. You have to vet your counterparties very carefully here as schemes and frauds are enormous. As a legit company it took us quite a while to convey legitimacy and foster trust.
I run a professional services firm in Shanghai catering to job seekers. Reach out to me by my profile email. TIA.
