French proofing tools

The English version of Office should include proofing tools for French and Spanish, right out of the box (so to speak). If you go to the Review tab, click Language, and select Set Proofing Language, the dialog shows a check mark to the left of each language for which you have proofing tools:

If French has a check mark, you should be able to choose it to mark the selected text in the document as using French proofing.

If you mark text as French and Word displays a yellow banner below the ribbon, starting with the words Missing Proofing Tools, then you can download the proofing tools for free. Click the Download button on the banner, which opens your browser to a page at Office.com; select French and click Download. Then run the downloaded installer program.

Even if the banner doesn't appear, there's another way to get the proofing tools. On the Review tab, click Language and then Language Preferences. In the dialog, look at French in the list of languages. If the third column says Not Installed, click that item (it's a link) to start the browser and go to Office.com.