Mark Z taught himself Chinese, did a Q & A solely speaking Chinese, and spoke strictly Chinese with Xi.

Said he is self-taught with help from his wife.

https://slate.com/business/2015/09/facebook-ceo-mark-zuckerberg-spoke-only-mandarin-with-chinese-president-xi-jinping.html

https://www.businessinsider.com/mark-zuckerberg-taught-himself-mandarin-2014-10

https://www.businessinsider.com/mark-zuckerbergs-mandarin-seems-to-have-vastly-improved-2015-2