A family of Microsoft word processing software products for creating web, email, and print documents.
This was driving me crazy, but I finally stumbled across the solution through a web search.
The suggestion I found, which finally got this working for me, was to select the entire contents of the document (most easily with Ctrl + A), and then changing the proofing language. (Not sure if it was important or not, but when I did this, I unchecked the box for "Do not check spelling or grammar," which appeared to be applying to a portion of the document, and also chose to "Set as default.")
Hope this helps!