How to change spelling library in Mac OS X with a shortcut?
You can set the Language spelling to look for spelling in multiple languages.

My example is English and Spanish.

In the Keyboard System preferences go to the Text tab.

Set the Spelling dropdown to: Automatic by Language

Then opening the Spelling dropdown go down to Set Up…



Drag your two (or more) preferred Languages to the top. And then hit the Done button.



Thats it.

Spelling will now recognise the language you are working in.