Normally we can change the language slug without loosing the translations... when this is done from the Polaylang languages panel. But here it was an unexpected way of changing it.
For the future, and waiting for the WordPress bug correction, I will prevent the creation of a menu with a language name. Starting from 1.1.3, the menu will be automatically renamed "Italiano-menu". That's a bit off-hand, but from far better than loosing translations and better than a notice that at least one person would not read.