My grandmother doesn't forget how to speak English.
Seriously - my great-grandmother brought my grandmother and her older brother to Italy to visit relatives when Mussolini rose to power. All the ports were closed for some time. My grandmother - a little girl at the time - was able to speak both English and Italian when she went there, but after being held up so long in Italy, she forgot how to speak English. It took her months to re-learn it when she returned.
Of course, this probably isn't the kind of event you're looking for.
I think this is an interesting idea. I'm wondering - if Mussolini doesn't rise to power - does Hitler even follow him? That is, would Adolf Hitler still be inspired to take up fascism if he doesn't have Mussolini's government as a successful model to look to?