Here is some basic concept you may need to understand: The language (Both in natural and programming) choice of a project is usually depends on the interests of the main contributors. The main contributors will select the one that is best for them, and make improvement base on that.

Since the project is mainly done by Tencent, a Chinese company, it just make since for the project to use Chinese. I see no point for their team members to communicate with one and other by writing documents in a second language everyday.

Also, they've provided English document if you want to read, and the document is not bad. If you don't like the document, then send them some PR.