For example, when visitors come from america the language auto change to English, while chinese visit my site, they will see chinese. I only found the options let visitors change the languages themeseves. By the way, I only buy one license, can I install two separate amembers in one site(one domain name), but two different directories for two different languages?
This should happen automatically. amember will check accepted language header that user's browser send to web server. (but of course this will work if you have all languages enabled) Sure you can do this.