A family of Microsoft word processing software products for creating web, email, and print documents.
In your Word document, enter an obvious misspelling and then select that word, only. Visit the Language dialog box (Review > Language > Set Proofing Language) and see if the correct proofing language is selected. Also, is the option "Do not check spelling or grammar" selected or cleared?
Based on the results of the testing suggested in the previous paragraph, further action may be required.
For example, you may have to select all content and (re-)apply the correct language and also ensure that "Do not check spelling or grammar" is cleared for all text.