Use xeCJK for fontenc as default with XeLaTex and Chinese docs

[zhsj]

When building Chinese translation of CPython docs. The only valid option I tried out is using xeCJK as the fontenc and using xelatex.

A simple project created by sphinx-quickstart, with some Chinese words(like 测试) in source/index.rst, will fail the make latexpdf command.

Howerver the following command works,

make latexpdf SPHINXOPTS='-D latex_engine=xelatex -D latex_elements.fontenc=\\usepackage{xeCJK} -D latex_elements.inputenc='

Although I still get warning like no Babel option known for language 'zh-cn', but the PDF looks good.

Could you set these as default for Chinese documents?

xeCJK will use Fandol font as default, both are Tex packages

https://ctan.org/pkg/xecjk
https://ctan.org/pkg/fandol

[zhsj]

Peviously the CPython docs are built with -D latex_engine=xelatex -D latex_elements.fontenc= -D latex_elements.inputenc=, the command will succeed, but the PDF doesn't contain any Chinese characters. I don't know much about latex, but I recall that I have used xeCJK before...

[tk0miya]

Thank you for the pointer. Okay, let's follow the python-doc's setting.

refs: #1119

[tk0miya]

Note: Sphinx expects the joint character of language is _ (underscore). So zh_CN is better for this case. But it does not resolve the warning.

[tk0miya]

Fixed by #6398.
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