Comment 21 for bug 207198

Wrong Sans-Serif config causing Chinese font display error in Adobe Flash Player and some other GTK programs

It can be solved if /etc/fonts/conf.avail/69-language-selector-zh-tw.conf is configured properly:

=== Before Edit ===
  <test qual="any" name="family">
   <string>sans-serif</string>
  </test>
  <edit name="family" mode="prepend" binding="strong">
   <string>Bitstream Vera Sans</string>
   <string>DejaVu Sans</string>
   <string>WenQuanYi Zen Hei</string>
   <string>AR PL UMing TW</string>
   <string>AR PL ShanHeiSun Uni</string>
   <string>WenQuanYi Bitmap Song</string>
   <string>AR PL UKai TW</string>
   <string>AR PL ZenKai Uni</string>
  </edit>
=== == ===

**** After Edit ****

 <test qual="any" name="family">
   <string>sans-serif</string>
  </test>
  <edit name="family" mode="prepend" binding="strong">
   <string>WenQuanYi Zen Hei</string>
   <string>AR PL UMing TW</string>
   <string>AR PL ShanHeiSun Uni</string>
   <string>WenQuanYi Bitmap Song</string>
   <string>Bitstream Vera Sans</string>
   <string>DejaVu Sans</string>
   <string>AR PL UKai TW</string>
   <string>AR PL ZenKai Uni</string>
  </edit>
************************

Edit "/etc/fonts/conf.d/49-sansserif.conf" could also solve the problem:

Replace all "sans-serif" with "sans serif"