I fixed it! As someone here suggested, I checked all my styles and several of them had Spanish as the language setting. I also found several where the spell check option was set to Do not check spelling.

On the Home ribbon, select the style, then right-click and select Modify. In the Modify Style dialog box, click Formst > Language and then in the Language dialog box, select the language you want to use for proofing that style. You will need to reapply the modified style(s) manually, or at the bottom of the Modify Style dialog, click Automatically update.