How to change the language of dictionary in word for mac

My friend is using Microsoft Word for Mac (which is a total train smash). and we want to change the spell check from English (US) to English (UK).

This is what I have tried:

1. Make a spelling mistake otherwise you can't get to spelling options.
2. Click Tools > Spelling and Grammar > Options > Dictionaries
3. Change language to English (UK) and click OK
4. Type an English (US) word 'Categorize'
5. Click Tools > Spelling and Grammar - 'Spelling and Grammar check complete' i.e. it did not mark the work incorrect.

Solution 1:

Try this:

1. Select all the text in the document (keyboard shortcut should be +A)

2. Go to Tools / Language and select English (UK) (instead of English (US) as shown below):

   

3. Remember to click the Default button (agree to the prompt)

Note: I've found that even after doing this (on Word for Windows at least), it still considers certain US spellings as acceptable alternates for UK ones and refuses to mark them as incorrect.