Thank you for posting the instructions on how you started GParted. It would help if you had also provided details on which strings are not displayed in Italian.
Lacking this information I started GParted and it would appear that the strings for the stock toolbar items are not translated (e.g. "Delete").
Is this what you were referring to?
GParted 0.33 Main Window in Italian
The stock icons come directly from GTK2. For example the "Delete" toolbar button is configured in src/Win_GParted.cc - line 248. From looking at the code the text "Delete" is never mentioned so this text must be coming directly from the GTK2 stock. Unfortunately the text "Delete" no longer appears to be translated in newer distros, such as Debian SID which we use as the base for GParted Live.
Some further research indicates that the GTK2 stock icons are now deprecated. See Gtk.stock is deprecated, what's the alternative?
Currently GParted uses GTK2; however, work is in progress to migrate to GTK3. See Merge Request 9 - GParted GTK3.
Perhaps the problem can be addressed after the GTK3 migration.