The right way to do this is write a plugin which includes a layer that overrides that method and includes its local translation file.
If this gets too complicated, then there is always the quick and dirty:
1. In each of the translation directories create a qa-lang-zzz.php file like this (change zzz and aaa to whatever you want, but keep the changes consistent in later steps):
<?php
return array(
'aaa' => ' question(s) ',
);
2. Extend your theme and include this function in it:
public function ranking_count($item, $class) {
$separator = qa_lang_html('zzz/aaa');
$this->ranking_cell($item['count'] . $separator, $class . '-count');
}
And that's it. You can check the $item['count'] value and change the string based on whether the values matches 1 or not. That way you can decide to use the translated singular or the translated plural.