MODX uses lexicon files. The files used by MODX itself can be found in core/lexicon/. The files used by add-ons can be found in core/components/<component_name>/lexicon.
The i18n (internationalization) system uses a three-layered approach, a 'namespace', such as "core", the language code, such as "zh" and a topic, such as "welcome". The topic is actually the key part of the filename, such as "welcome.inc.php".
You can modify these lexicon files in the Lexicon menu entry under the Tools gear icon on the right of the main menu.
According to the announcement of the release of Revolution 2.3, a Chinese lexicon was added. This is the files found in core/lexicon/zh/. Apparently the translation was never completed, and has not made it into Revolution 2.4.0.
The installer also has a language selector at the very beginning, these files are in the setup/lang/ directory.
You can see the progress in translation at the official translation site,
https://crowdin.com/project/modx-revolution. Perhaps you can help work on the translations, or know someone who can.