A family of Microsoft word processing software products for creating web, email, and print documents.
In Word, language is a character property. The quickest fix will be to just select all the text in your document and re-apply your flavor of English via the Language dialog box (Review tab > Language group > Language > Set Proofing Language). In the dialog box, choose "Selected text," specify the appropriate language, click to clear the boxes for "Do not check spelling or grammar" and "Detect language automatically" and finally click OK (specifically do NOT click Set As Default, because this is about modifying the selection).
For everything on proofing in Word, see Suzanne S. Barnhill's article at http://wordfaqs.ssbarnhill.com/MasterSpellCheck.htm.