Given the pattern:

 Monospace-10:lang=ar

And the string "ABC" to display with it. You have in your fonts.conf

 <alias>
    <family>monospace</family>
    <prefer>
        <family>Andale Mono</family>
    </prefer>
 </alias>

On your system, you have a single font covering Arabic and Roman, "KacstBook", 
thatisn't even close to monospace.

The FcFontSort results will be: KacstBook, Andale Mono, resulting in
"ABC" rendered with KacstBook. My contention is that the ordering should
be Andale Mono, KacstBook instead.

I don't have a good idea about how this would be implemented. 

What we need is that Lang match vs. Lang no match reorders within
weak family matches, but any weak family match beats no weak
family match without regard to Lang.

Possibly what could be done is to allow instead of two slots in the
value array per match element, 3 slots.

 weak strong any

And makethe matchers in addition to returning a numeric result,
return whether there was any result at all, and if so, we stick
constant value in the any slot.

So, then you could order things in the value array as:

 family strong
 family any
 lang 
 family weak

Ugly, but it's a real problem. Without something like this, it's
really hard to support a meaningful "monospace" alias in a language where 
monospace fonts don't make sense (like Arabic)