This application allows you to activate the Spanish language if it does not exist on your phone

Works without any intervention on your part until Jelly Bean (4.2.2)

Since Jelly bean (4.2.2) this application will need CHANGE_CONFIGURATION permission to be able to change the system language

If you know how to use adb command from Android developer tools, you can download it and manually grant permission to this app by connecting your smartphone to your laptop with a cable, using this command through your laptop:

adb shell pm grant com.jkdevandroid.languagepanish android.permission.CHANGE_CONFIGURATION

Information: check google for more details

If your smartphone has root access, the app can automatically get permission for you

From Android 7 Nougat, the application will request a new permission to write the system configuration.

Some smartphone manufacturers remove unused languages ​​from the system entirely, in which case nothing can be done.