A family of Microsoft word processing software products for creating web, email, and print documents.
Select all the text in an affected document (Ctrl+A). Open the Language dialog box (Review tab > Language group > Language > Set Proofing Language), pick your preferred language and ensure that "Do not check spelling or grammar" is cleared. Confirm with OK (do NOT click Set as Default, because you are changing the selection, not the default setting, in this case).
Does that fix the problem?
For more on language formatting in Word, see http://wordfaqs.ssbarnhill.com/MasterSpellCheck.htm .