Find the file /resources/lang/en/storeDashboard.php
All the translations for the English Language are present in this file.
Create a new folder inside /resources/lang
This new language folder name should compile with ISO 15897.
If you need to create a Spanish translation, follow the steps:
Create a folder named "es" inside /resources/lang
Copy the file "storeDashboard.php" from "en" folder to your new "es" folder.
Open the newly copied file and edit language as you wish.
Save the file
Now this language will be available in the Store Owner's Dashboard Language drop-down menu in the Header (Top-Right Corner of the Store Owner's Dashboard)