FIND:

return abs($diff_start_event) . ' ' . __('days left', 'cosmotheme');

and

return abs($diff_end_event) . ' ' . __('days left', 'cosmotheme');

CHANGE IN:

return 'Tra ' . abs($diff_start_event) . ' ' . __('giorni', 'cosmotheme');

and

return 'Tra ' . abs($diff_end_event) . ' ' . __('giorni', 'cosmotheme');