>> What we have no is ‘this’, 'this' and `this'.  What we could add is 
>> `this´.
>
> Why just that one, though?  There are several more quotes in the 
> relevant Unicode block, why not add all of them?
>

Indeed, all of them could be added.  With two options, then: 
text-left-quote-style and text-right-quote-style (and two similar options 
for buffers).

>
> They definitely fit better for this task than ´, which is not even a 
> quote character, and isn't classified as punctuation.
>

That depends on each one's preferences.  And ` is also not a quote 
character, it's a grave accent; to some at least it might seem natural to 
use an acute accent to match a grave accent.