99-local.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
        <!-- serif -->
        <match target="pattern">
                <test qual="any" name="family">
                        <string>serif</string>
                </test>
                <edit name="family" mode="prepend" binding="strong">
                        <string>Contra</string>
                </edit>
        </match>

        <!-- sans-serif -->
        <match target="pattern">
                <test qual="any" name="family">
                        <string>sans-serif</string>
                </test>
                <edit name="family" mode="prepend" binding="strong">
                        <string>PT Sans Caption</string>
                        <string>WenQuanYi Micro Hei</string>
                </edit>
        </match>

        <!-- monospace -->
        <match target="pattern">
                <test qual="any" name="family">
                        <string>monospace</string>
                </test>
                <edit name="family" mode="prepend" binding="strong">
                        <string>Hack</string>
                        <string>WenQuanYi Micro Hei Mono</string>
                </edit>
        </match>

        <!-- Adobe Flash issue -->
        <match target="pattern">
                <test qual="any" name="family">
                        <string>AR PL UKai CN</string>
                </test>
                <edit name="family" mode="prepend" binding="strong">
                        <string>Droid Sans Fallback</string>
                </edit>
        </match>
</fontconfig>

gistfile1.txt

gslin@GSLIN-HOME1404 [~] [21:11/W4] fc-match -s sans | head -n 5 
PT_Sans-Caption-Web-Regular.ttf: "PT Sans Caption" "Regular"
wqy-microhei.ttc: "文泉驛微米黑" "Regular"
DejaVuSans.ttf: "DejaVu Sans" "Book"
DejaVuSans-Bold.ttf: "DejaVu Sans" "Bold"
DejaVuSans-Oblique.ttf: "DejaVu Sans" "Oblique"
gslin@GSLIN-HOME1404 [~] [21:11/W4] fc-match -s sans-serif | head -n 5
PT_Sans-Caption-Web-Regular.ttf: "PT Sans Caption" "Regular"
wqy-microhei.ttc: "文泉驛微米黑" "Regular"
DejaVuSans.ttf: "DejaVu Sans" "Book"
DejaVuSans-Bold.ttf: "DejaVu Sans" "Bold"
DejaVuSans-Oblique.ttf: "DejaVu Sans" "Oblique"

gslin@GSLIN-HOME1404 [~] [01:32/W4] fc-match -s sans-serif:lang=zh-tw | head -n 5
PT_Sans-Caption-Web-Regular.ttf: "PT Sans Caption" "Regular"
wqy-microhei.ttc: "文泉驛微米黑" "Regular"
DroidSansFallbackFull.ttf: "Droid Sans Fallback" "Regular"
DejaVuSans.ttf: "DejaVu Sans" "Book"
DejaVuSans-Bold.ttf: "DejaVu Sans" "Bold"
gslin@GSLIN-HOME1404 [~] [01:32/W4] fc-match -s sans-serif:lang=zh | head -n 5 
PT_Sans-Caption-Web-Regular.ttf: "PT Sans Caption" "Regular"
wqy-microhei.ttc: "文泉驛微米黑" "Regular"
DroidSansFallbackFull.ttf: "Droid Sans Fallback" "Regular"
DejaVuSans.ttf: "DejaVu Sans" "Book"
DejaVuSans-Bold.ttf: "DejaVu Sans" "Bold"
gslin@GSLIN-HOME1404 [~] [01:32/W4] fc-match -s sans-serif | head -n 5 
PT_Sans-Caption-Web-Regular.ttf: "PT Sans Caption" "Regular"
wqy-microhei.ttc: "文泉驛微米黑" "Regular"
DejaVuSans.ttf: "DejaVu Sans" "Book"
DejaVuSans-Bold.ttf: "DejaVu Sans" "Bold"
DejaVuSans-Oblique.ttf: "DejaVu Sans" "Oblique"