Substitute Linux default font family for Arabic with Helvetica World. Save as /etc/fonts/conf.d/99-arabic.conf

99-arabic.conf

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- $XDG_CONFIG_HOME/fontconfig/fonts.conf for per-user font configuration -->
<fontconfig>
<match>
        <test name="lang" compare="contains">
                <string>ar</string>
        </test>
        <test name="family" compare="not_eq">
                <string>monospace</string>
        </test>
        <edit name="family" mode="prepend">
                <string>Helvetica World</string>
        </edit>
</match>
</fontconfig>