You must

• install the core installation is Spanish
• translate you theme strings by hand
• translate the plugins that you are going to use (Loco Plugin)

Do you want to translate WordPress plugins in your language?

Here's a step by step guide on how to easily translate a WordPress plugin in your language. https://www.wpbeginner.com/wp-tutorials/how-to-translate-a-wordpress-plugin-in-your-language/  

when you finish the translation you must inform the vendor and send the files OR STOP UPDATING FOR THIS PLUGIN with another plugin.

If the Author has no the translation the next update of plugin will delete your work
  and always backup