A family of Microsoft word processing software products for creating web, email, and print documents.
I wonder if you have an add-in with Word that is blocking the 1033 (US English) version for spelling. I know that if you have a 3rd party dictionary app installed upon Office installed, sometimes, the dictionary can be disabled.
Try reproducing this in safe mode - Start/Run (or Search) and type this in:
winword.exe /a
Then try to reproduce the issue. If it still happens, it's not likely an addin. If it does work, then you have an add-in conflict.