The WordPress slugs may indeed be missing the accents, but I am not sure why. It might be that your database isn’t encoded to your language?

With some trepidation if you have not done it before. You need access to the database first. If you are not sure how to do this, the ARI Adminer plugin will let you do this from within WordPress (but do not leave it installed; use it then remove it). See this screenshot:

https://www.screencast.com/t/6SS8rkv0bG

The first time you run Adminer it sets up a crypt key. Then you will see this at the bottom of your admin sidebar menu:

https://www.screencast.com/t/ClYHPCiole

When you run Adminer (I do this in a new window, the grey button):

https://www.screencast.com/t/m9FCeUJM

You will now see your database, it will look like this:

https://www.screencast.com/t/QUVGGh9Rn

In the list of tables, you can see collation, which is the language.

NOTE. Some hosts do not allow database access from within WordPress, and so ARI Adminer will throw an error.