change project Language

I have finished a non-fiction book that I have written in Spanish (Spain).
I did this by just opening a new project and changing the binder and texts to Spanish.
I see how you can change the interface to another Language but I don’t want to this. (I work mainly in English)
I would like to be able to do a spell-check in Spanish but can’t see how to change over to Spanish.
Can anyone tell me how to do this?

It’s a macOS setting. See Prefs/Keyboard/Text
From the Spelling dropdown, select Set Up…

Thanks. Very helpful