How can I customize the right click menu?
You can't change what the Context menu item does in so far that it is fixed to look up a word in the macOS dictionary.
You can't even change it to use a specific dictionary.
You can, however, make some changes. The look up that is done is according to the preferences of the Dictionary app and it is possible to change the dictionary used to another that Apple makes available, such as switching from British to American dictionaries.
To do this open the Dictionary app and select Preferences... in the 'Dictionary' menu and you will see a list of dictionaries that can be enabled or disabled as you wish.