Changing the language Mac OS X uses for spelling.
Within Mac OS X 10.9.5 (Mavericks) and above you should be able to change the spelling language to be independent of the overall system language by going to “System Preferences… -> Keyboard” and then in that panel, select “Text” and within that panel you should see a “Spelling:” pulldown near the right-hand side of the panel that looks like this:
The default is “Automatic by Language”—which causes the behavior you are experiencing—so just change that value to be whichever flavor of English you prefer to use.
Not too sure about newer versions of Mac OS X, but it seems you can only choose one language for spelling in Mac OS X 10.9.5 (Mavericks).
Changing the language Microsoft Word uses for spelling.
And while this is only connected to spell checking within Word, according to Microsoft themselves, you can set the “proofing language” independently of the whole system by following these instructions:
To set the proofing language for text and check your spelling in your
document, do the following:
Select the text that you wants to check.
On the Review tab, in the Language group, click Language, and the click Set Proofing Language Word Ribbon Language button.
In the Language dialog box, verify that the correct language is associated with the text. If it is not, select the language that you
want to use for the dictionary and other proofing tools.
If the language is preceded by the Spelling & Grammar icon, it
means that proofing tools, such as spelling checker, are installed for
that language. If the language is not preceded by the Spelling &
Grammar icon, the dictionary for that language is not available and
you cannot check the spelling and grammar for that language.
Complete steps 1-3 for each section of text that is in a different language.