Hi,
Thanks for contacting us. You do not need to edit the article override for this.
First, make sure Show Publish Date is enabled in the article, menu item, or global Article options.
Then go to:
System → Manage → Language Overrides
Select Spanish – Site and create the following override:
Language Constant:
COM_CONTENT_PUBLISHED_DATE_ON
Text:
Fecha de publicación: %s
Make sure you keep %s, because Joomla replaces it with the actual published date.
Save the override and clear the Joomla and browser cache. If it still does not appear, your template may have its own com_content article override that is hiding the published-date information.
-Regards.