Hi,
You can edit the view "product / quantity" and add an if condition around:
echo '<span class="hikashop_product_stock_count">'.JText::sprintf('X_ITEMS_IN_STOCK',$this->row->product_quantity).'</span><br/>';
Like:
if($this->row->product_quantity == 1)
echo '<span class="hikashop_product_stock_count">'.JText::sprintf('X_ITEMS_IN_STOCK',$this->row->product_quantity).'</span><br/>';
else
echo '<span class="hikashop_product_stock_count">'.JText::sprintf('X_ITEM_IN_STOCK',$this->row->product_quantity).'</span><br/>';
And add the translation "X_ITEM_IN_STOCK" in the language file with the desired translation.