This issue is still happening every time I update Flarum.
If I forget to change the Flarum default language from Italian to English then deactivate the Italian translation, activate again and then set again Italian as default language in the basic settings, the English language that is my secondary forum language is broken.
I set this topic solved but is not and I cannot remove the best answer so the solved status.
I may need to report this issue as BUG on GitHub.