Is it possible to change the default language to Spanish (or another language)?
To change a default language, perform the following:
Go to Plesk > Tools&Settings > Plesk Appearance > Languages > Available languages .
Select needed language pack and then click Make Default button.
Log out from Plesk.
Log in to Plesk.
Also, this can be done using CLI:
# plesk bin locales --set-default es-ES