Firstly I do not have
File > Options > Custom Dictionaries
File > Options >[Recent | OneDrive | This PC | Add a place | Browse]



Secondly when I insalled office 2016, I set the languge to English (UK). So it is not the built in American custom dictionary. I have tested it out with words spelt diferently in Amercan.

Do I have add an additional dictionary as well as the American and UK dictionary.
Also this is a home computer not connected to any network.


Also when you add to dictionary, you definatly do not add to custom.dic. Word creates a seperate file for words added.

File > Options > Proofing > Custom Dictionaries
Not sure what the second one is about
File > Options > Language is another place to be looking.
Have you looked at Suzanne's article? Mastering the Spelling Checker by Suzanne Barnhill, MVP
Have you looked at my article? Spelling Checker Does Not Work! - Proofing Language Keeps Changing - Solutions
What is your language setting for Windows?
You do not have to add anything.
Sorry, you add to the custom dictionary default.dic or another one that you create.