I'd like to buy a new model iPhone 6s 16 GB ("Rose Gold" color) and use it in China.
First off, the iPhone 6S is a great choice. It's the gold-standard of all phones, but whatever you do, DON'T buy the 16 GB model. This isn't an iPhone 3GS or a cheap Android phone, it's a brand new iPhone 6S and it shoots 4K videos. Formatted, with iOS on it, you should have less than 12 GB to work with. Put 2 HD movies on it, and your just about out of storage. Get at least the 64 GB model, it's way worth the $100, IMO, plus when you go to sell it for an upgrade in a few years, you'll get back most of that $100, anyway. Me? I've already bought a 128 GB iPhone 6S on Verizon.
When purchasing it in Apple Store, what options should I choose so the phone will bear no service contract which is useless for me?
Not a lot, other than get the iPhone on T-Mobile, which should be available to buy outright ($750 for 64 GB model) and will come SIM-free and without a contact, just make sure that it will work on whatever carrier you wish to put it on in China.
How to make sure it is "unlocked" so it can be used with a local service company in China? What sort of SIM card does this iPhone require for it to be compatible?
Apple doesn't sell unlocked iPhone 6Ss, yet. I asked an Apple rep about this and they said maybe in a few months, but there is the very small chance that your carrier doesn't require the iPhone to be unlocked, like Straight Talk, but probably not in China.
Could the interface language be set to Chinese language?
Yes. China and the US are Apple's biggest customers.