See Mastering the Spelling Checker.

And what, say you, has that got to do with it? I suspect that it is the proofing language setting that is causing your problem. It is using the French version of AutoCorrect because it believes the language being used is French. It should also mark any word that is not in the French dictionary as misspelled. It may be set to not check spelling or grammar.

Start with a copy of your document.
Press Ctrl+A, then
Under the Review tab click on the Language button and select Proofing Language.

If the box at the bottom to not check spelling or grammar is checked, uncheck it.
Take a look at the language that has been chosen. You could OK/exit your way back to your document and see if things are now marked as misspelled.

Otherwise, pick the correct language.

If that helped, you will also need to address style settings. If it helps, write back and we will try to do something about them. If it did not help, I am on a wrong track.