Have you chosen Catalan as a language in System Preferences > Language & Region?
You could also achieve this by choosing Catalan in System Preferences > Keyboard > Input Sources. This automatically adds Catalan as a language choice as above.
Also put a tick i the box saying 'Show input menu in menu bar'. From that menu bar item you can toggle between Catalan and English keyboard - and whatever else you choose. Bear in mind though that some keys may behave differently - you'll have to experiment with that yourself.
Now when you choose Catalan from the menu bar you will be typing in Catalan and can double click on a word, right click and find the definition and spelling.
I can't verify however if this will also work as an autocorrect tool. Try it and let us know.