How to change the default dictionary on iPhone & Mac?
In order to change the priority on your Dictionary App, go do Dictionary > Preferences.

Choose the dictionaries you want to be activated — assuming you have them installed — and drag them to choose the priority, *from top (highest priority) to bottom.

Note that if you look for a word in a certain language, dictionaries that don't have that word won't appear in the search results.



Change localization on your Mac:


Go to System Preferences > International > Languages.
Click on "Edit list" to add/remove the dictionaries.
Drag the dictionaries in the list itself to sort the priority which works from top to bottom.