$XDG_CONFIG_HOME/fontconfig/fonts.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
    <match>
        <test name="lang" compare="contains">
            <string>ja</string>
        </test>
        <test name="family">
            <string>sans-serif</string>
        </test>
        <edit name="family" mode="prepend">
            <string>M+ 1p</string>
        </edit>
    </match>
</fontconfig>