It is possible. You will have to run festival with the "--language italian" parameter. I use another TTS program so I'm not sure, but I think "--pipe" is also a good idea.
You could run festival in server mode with the "--server" switch. If you do so, the whole command to run it would be
festival --server --language italian --pipe
You'll also have to turn the "Sound server running (Linux)" option on in the plugin settings.
You needn't run festival as a server, but then you'll have to pass these executables differently.
If adding the options to the "Path to festival" does not work, try creating a symlink to the festival binary with the options, and entering the symlink into the "Path to festival" field. The Festival executable is most likely in /usr/bin. If you can't find it, you could try
$ locate festival
EDIT: use a script instead of a symlink
Hope that helped