Hi, I know the answer's a little late, but I think the problem you are facing is that the translation file should be:
[prestawebroot]/app/Resources/translations/es-ES/ShopFormsLabels.es-ES.xlf
At least that's what worked for me on 1.7.3
Hope it helps someone.
Tried this solution in PrestaShop 1.7.4
The file is called modules/ps_mainmenu/ps_mainmenu.php search around line 717
Unfortunately there appears to be some caching in this file, so that the changes are only reflected after categories are updated.
This basically makes the proposed solution unworkable.