If the language selected in the page property for a page is English and the user select a language from the preference as Italiano the UI in the AEM(example component options like Edit, Annotate, Delete, new, etc) are still in English and not in the Italiano.
Language of a page, either in path or page property (jcr:language) have no impact on UI language. UI language is solely driven off whatever the user sets as UI language, in user preferences.
For example below, English page (/en/ in path, jcr:language set to English) in German UI
Language of a page, either in path or page property (jcr:language) have no impact on UI language. UI language is solely driven off whatever the user sets as UI language, in user preferences.
For example below, English page (/en/ in path, jcr:language set to English) in German UI