I welcome people who want to translate Ren'Py.
To translate the Ren'Py launcher, you want to open the launcher as a project. To do that go into preferences, and choose "Open launcher project". You can then click the "Generate Translations" button to generate a translation for your language.
To translate a template, go into the templates directory, and copy "english" to "chinese". Then go to the launcher preferences, check "Show templates", and go back to the front page. You'll see the various templates. Choose your language template, "Generate Translations", and generate for the None language, which is what Ren'Py uses by default.
When done, send it to me, ideally via a git pull request.
There aren't many rules for translations - do a good job, I guess, and try to keep up with Ren'Py development. Especially, keep an eye out for pre-releases, and update the translation when a new pre-release comes out. I don't hold up releases for translations, but a significant update will merit a bugfix release.
For Chinese in specific, I think we probably want to have "simplified_chinese" and "traditional_chinese" languages - right? Those would be different translations, and not just a font change. I'm not super familiar as to how that works.
The documentation is more complex - there's a project to translate it on launchpad, here:
https://launchpad.net/renpy-doc
I don't actually know how to use it, however.