Default sans-serif font which is used in system, doesn't have some characters from Middle-Eastern languages. So I want to set a fallback font to be used when some symbols are missing.
AFAIK, it's related to fontconfig and fonts.conf files. Here is my current fonts.conf file:
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
<alias>
<family>sans</family>
<prefer>
<family>FONTNAME</family>
</prefer>
</alias>
</fontconfig>
where FONTNAME is the font which have all symbols. It works, but after setting this, the default font disappeared and whole system use FONTNAME as default sans font. (Note that, FONTNAME has latin characters too.)