lots of questions
and the part with the Atmega16u2. As i understood, it's only a USB to UART bridge
so i can substitute it with a FT232 or similar or use a MAX232 and connect to a serial port, right?
will i be able to program the Atmel from Arduino IDE (as long as i upload the bootloader on it with an atmel programmer)?
yes, the old arduinos were serial only, and as far as both ends are concerned its serial, so its all good
Will it work with no handshake (i mean only with RX and TX) or i need to wire also other signals (RTS and CTS)?
it will if you manually reset the board when you want to program it (wait till it says uploading and hit the reset button) otherwise you will need a control line, I dont remember which one reset is on off hand
Moreover, does the core part require other things rather than the microcontroller, the crystal at 12MHz, the bypass capacitors and the resistor on RESET? I mean, with Atmega328, 12MHz resonator, one 100nF capacitor and a pullup resistor i can have a working arduino uno?
sort of, the 328P runs on a16Mhz crystal, and you will need a couple caps for that, or you can get a ceramic resonator which is a handy 3 pin package (2 xtal 1 ground in the middle).
you might want a reset button, you will need it if your going to manually reset it for programming. but otherwise yea the main functionality of arduino is all on the 328P
also the pins are not mapped directly, so here is a handy charthttp://arduino.cc/it/Hacking/PinMapping168
And.. is there an italian forum too or the only supported language is english?
yes, there are a few language sections in the forum http://arduino.cc/forum/index.php/board,34.0.html