Solved. Tl;dr: it's not enough to set compiler to be xelatex instead of pdflatex; one has to include package xeCJK as well, but for where it should be, see below.
Edit: the below can also be achieved by setting these pandoc template variables:
---
CJKmainfont: STSong
CJKoptions:
- BoldFont=STHeiti
- ItalicFont=STKaiti
---
Hello 你好
In my case, pandoc reads a .md, converts it to be a .tex, and call compiler to compile that to be a .pdf.
Thus, in theory, what I can do normally with a tex-like compiler, can be done with pandoc as well --- it is only that I have to specify required template.
The pandoc calls its own latex template, which we can cat in the terminal by a prewritten command:
pandoc -D latex > default.latex
This essentially copies a new file default.latex to . (current directory).
It is this I now modify.
Append this option to pandoc when compiling:
--template=my-directory/my-template.latex
In the past I type Chinese characters (or more generally, CJK), I use a template beginning with
\documentclass[12pt]{article}
\usepackage{xeCJK}% use Latin font whenever possible
\usepackage{fontspec}% set Chinese fonts, as follows
\setCJKmainfont[BoldFont=STHeiti,ItalicFont=STKaiti]{STSong}
\setCJKsansfont[BoldFont=STHeiti]{STXihei}
\setCJKmonofont{STFangsong}
% .... whatever xeCJK commands you use
The fonts should be those your system permits; these shown above are shipped with mac.
But when I simply pasted this into the pandoc-provied template, there were many cryptic error messages
option clash for package XXXX....
This was because the pandoc-proviede template already defined xeCJK. Indeed, search these lines:
$if(CJKmainfont)$
\usepackage{xeCJK}
\setCJKmainfont[$for(CJKoptions)$$CJKoptions$$sep$,$endfor$]{$CJKmainfont$}
$endif$
These lines (quoted part) should be replaced by
\usepackage{xeCJK}
\setCJKmainfont[BoldFont=STHeiti,ItalicFont=STKaiti]{STSong}
% .... and so on, whatever you call from xeCJK
i.e., delete if, so that xeCJK is always executed; otherwise, xeCJK line will not be copied to the intermediate .tex file.
And also delete \usepackage{fontspec}, because it is called by pandoc by default, otherwise error message occurs for packages are called twice in the intermediate .tex file.
acknowledgement: [1] [2] [3]
Sorry I didn't keep track of every websites from which I referenced, but none gets it all right anyway, or are outdated.
Of most help is mb21, who suggested in the comment that I output .tex to debug, after which I found xeCJK was not included.
I have spent some 10+ hrs on this issue, but from now on I can happily type Chinese in a markdown file.
I have wrote this down for poor posterity's sake.
