It is because your PC is missing some files that are required for your language so you need to reinstall those missing files from the office source. How to do that?
1.Open your Control Panel
2. navigate to \All Control Panel Items\Programs and Features\
3. you will see the interface that allows you to "Uninstall or Change a Program".
4. now, select and right click on the version of your "Microsoft Office". Then select "Change". Now you will be able to reinstall the missing files of your preferred language.
5. select "Add or Remove Features". Continue.
6. On "Office Shared Feature", select and install the Proofing Tools". That is it. Done.
7, After you install this Proofing Tools, you should see the red underlined misspelled words when you are tying words.