Try: Review> Language> Set Proofing Language ( and make sure "Do not check spelling or grammar" is unchecked).
You may also check Review>Language> Language Preferences> Proofing> and at the bottom of the window make sure the Exceptions for check boxes are unchecked.
For other solutions try:
- Microsoft Support (2016). Check spelling and grammar in a different language in Office 2007. Retrieved from https://support.office.com/en-us/article/Check-spelling-and-grammar-in-a-different-language-in-Office-2007-de22dd18-2766-4243-8a98-8963c6726ee4