1) Translate resource file
The default English resource file is Web\App_GlobalResources\Common.resx. You can make a copy of it and name it Common.[culture code].resx e.g. Common.zh-CN.resx for mainland China. And then translate the resource file.
2) Translate theme files
The default English theme files are located in Web\App_Themes\admin and Web\App_Data\Themes_Template\Default. Make a copy of these and put it the peer directory named as admin.[culture code] and Default.[culture code]. E.g. Web\App_Themes\admin.zh-CN and Web\App_Data\Themes_Template\Default.zh-CN
Open up default.skin file and find string like "text=" e.g. text=Next. Translate the string after "text=". E.g. "text=下一步". And then you need to URL encode the localized string "下一步" to something like "text=%e4%b8%8b%e4%b8%80%e6%ad%a5" because the unicode must be URL encoded to be shown correctly in URL.
3) Enable the culture
Open up Web\Admin\Site\Controls\Languages.ascx.cs and your newly added culture into the following array.
string[] langs = { "en-US", "zh-CN" };