Example fontconfg file

fonts.conf

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'urn:fontconfig:fonts.dtd'>
<fontconfig>
 <!-- Default serif font -->
 <alias binding="strong">
  <family>serif</family>
  <prefer>
   <family>Roboto Slab</family>
  </prefer>
 </alias>
 <!-- Default sans-serif font -->
 <alias binding="strong">
  <family>sans-serif</family>
  <prefer>
   <family>Greycliff CF</family>
   <family>Atkinson Hyperlegible</family>
  </prefer>
 </alias>
 <!-- Default monospace font -->
 <alias binding="strong">
  <family>monospace</family>
  <prefer>
   <family>Cartograph CF</family>
  </prefer>
 </alias>
 <!-- Default system-ui font -->
 <alias binding="strong">
  <family>system-ui</family>
  <prefer>
   <family>Lato</family>
  </prefer>
 </alias>
 <!-- Serif CJK -->
 <!-- Default serif when the "lang" attribute is not given -->
 <!-- You can change this font to the language variant you want -->
 <match target="pattern">
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK SC</string>
  </edit>
 </match>
 <!-- Japanese -->
 <!-- "lang=ja" or "lang=ja-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ja</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK JP</string>
  </edit>
 </match>
 <!-- Korean -->
 <!-- "lang=ko" or "lang=ko-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ko</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK KR</string>
  </edit>
 </match>
 <!-- Chinese -->
 <!-- "lang=zh" or "lang=zh-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK SC</string>
  </edit>
 </match>
 <!-- "lang=zh-hans" or "lang=zh-hans-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hans</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK SC</string>
  </edit>
 </match>
 <!-- "lang=zh-hant" or "lang=zh-hant-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK TC</string>
  </edit>
 </match>
 <!-- "lang=zh-hant-hk" or "lang=zh-hant-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant-hk</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK HK</string>
  </edit>
 </match>
 <!-- Compatible -->
 <!-- "lang=zh-cn" or "lang=zh-cn-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-cn</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK SC</string>
  </edit>
 </match>
 <!-- "lang=zh-tw" or "lang=zh-tw-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-tw</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK TC</string>
  </edit>
 </match>
 <!-- "lang=zh-hk" or "lang=zh-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hk</string>
  </test>
  <test name="family">
   <string>serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>Noto Serif CJK HK</string>
  </edit>
 </match>
 <!-- Sans CJK -->
 <!-- Default sans-serif when the "lang" attribute is not given -->
 <!-- You can change this font to the language variant you want -->
 <match target="pattern">
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Japanese -->
 <!-- "lang=ja" or "lang=ja-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ja</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Korean -->
 <!-- "lang=ko" or "lang=ko-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ko</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Chinese -->
 <!-- "lang=zh" or "lang=zh-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hans" or "lang=zh-hans-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hans</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hant" or "lang=zh-hant-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hant-hk" or "lang=zh-hant-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant-hk</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Compatible -->
 <!-- "lang=zh-cn" or "lang=zh-cn-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-cn</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-tw" or "lang=zh-tw-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-tw</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hk" or "lang=zh-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hk</string>
  </test>
  <test name="family">
   <string>sans-serif</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Monospace CJK -->
 <!-- Default monospace when the "lang" attribute is not given -->
 <!-- You can change this font to the language variant you want -->
 <match target="pattern">
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Japanese -->
 <!-- "lang=ja" or "lang=ja-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ja</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Korean -->
 <!-- "lang=ko" or "lang=ko-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ko</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Chinese -->
 <!-- "lang=zh" or "lang=zh-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hans" or "lang=zh-hans-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hans</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hant" or "lang=zh-hant-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hant-hk" or "lang=zh-hant-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant-hk</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- Compatible -->
 <!-- "lang=zh-cn" or "lang=zh-cn-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-cn</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-tw" or "lang=zh-tw-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-tw</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- "lang=zh-hk" or "lang=zh-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hk</string>
  </test>
  <test name="family">
   <string>monospace</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <!-- System UI CJK -->
 <!-- Default system-ui when the "lang" attribute is not given -->
 <!-- You can change this font to the language variant you want -->
 <match target="pattern">
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- Japanese -->
 <!-- "lang=ja" or "lang=ja-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ja</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- Korean -->
 <!-- "lang=ko" or "lang=ko-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>ko</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- Chinese -->
 <!-- "lang=zh" or "lang=zh-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- "lang=zh-hans" or "lang=zh-hans-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hans</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- "lang=zh-hant" or "lang=zh-hant-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- "lang=zh-hant-hk" or "lang=zh-hant-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hant-hk</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- Compatible -->
 <!-- "lang=zh-cn" or "lang=zh-cn-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-cn</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- "lang=zh-tw" or "lang=zh-tw-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-tw</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- "lang=zh-hk" or "lang=zh-hk-*" -->
 <match target="pattern">
  <test compare="contains" name="lang">
   <string>zh-hk</string>
  </test>
  <test name="family">
   <string>system-ui</string>
  </test>
  <edit binding="strong" mode="append" name="family">
   <string>LXGW WenKai</string>
  </edit>
 </match>
 <!-- REAPER fonts patch -->
 <match target="pattern">
  <test name="prgname">
   <string>reaper</string>
  </test>
  <edit mode="assign" name="family">
   <string>MiSans VF</string>
  </edit>
 </match>
 <dir>~/.fonts</dir>
</fontconfig>

This file is in .config/fontconfig.
The main content is line 525.