I have been trying to localise the error messages presented when user information becomes an issue. In the code below, I tried to localise it to French but it simply didn’t work and the error just got printed in English.

 Auth.auth().signIn(withEmail: email, password: password) { authResult, error in
                if let errorResults = error {
                    Auth.auth().languageCode = "fr"
                    print(errorResults)
                } else {
                    self.performSegue(withIdentifier: "LoginComplete", sender: self)
                }
            }

(Before I get asked “Is this a duplicate” seemingly no-one has discussed this topic for Swift and anything similar does not have a sufficient answer.)