Hi Andre,
“Mandarin” refers primarily to spoken Chinese.
For written language, you generally decide between “simplified” and “traditional”.
I generally use the Chinese setting for simplified Chinese ( mainland China & Singapore ), and the Chinese (Taiwan) setting for traditional Chinese ( Taiwan, Hong Kong & Macau ).
The reason is that Webflow is currently using Amazon Translate and Amazon’s docs indicate two language codes for Chinese;
Webflow does not show these codes in the list, but once you select a locale, you’ll see the code appear in your locales page, and you can confirm that you have the right one for Amazon’s machine translate features.
ADDED: I see this question a lot so I’ve added a video here.