You can create the file ~/.config/fontconfig/conf.d/90-my-farsi.conf
and give it this contents as a starter:
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
<match target="pattern">
<test name="lang" compare="contains">
<string>fa</string>
</test>
<test qual="any" name="family">
<string>sans-serif</string>
</test>
<edit name="family" mode="prepend" binding="strong">
<string>Homa</string>
</edit>
</match>
<match target="pattern">
<test name="lang" compare="contains">
<string>fa</string>
</test>
<test qual="any" name="family">
<string>serif</string>
</test>
<edit name="family" mode="prepend" binding="strong">
<string>Homa</string>
</edit>
</match>
</fontconfig>
That file results in the Homa font (included in the fonts-farsiweb package) being selected at first hand when Persian is the selected language.
$ LANG=fa_IR fc-match
homa.ttf: "Homa" "Regular"
$