Good question and one which has been asked many time. All I can find on the net for an Alfa is this procedure (who wants to try it on a Maser?)
I checked on elearn, and country codes for 159 are the same. Procedure for manual setting of country code on 147 (and most probably same on 159):
UK = 2, Holland = 4 (no beeps)
The operating mode can be changed by diagnostic instrument or the following manual procedure:
set the ignition key to Key-On;
activate the hazard warning flasher
press the rear fog light control button and hold it down
using the ignition key, carry out the following sequence in strict order: Key-Off , Key-On , Key-Off, quickly in sequence, starting from the Key-On state, always holding down the rear fog light button.
at the end of the sequence, the siren emits 5 beeps to indicate the activation of programming.
release the rear fog light switch and press it again the same number of times as the desired mode (each press gives a feedback beep). so 4 times to set to Dutch
If the button is pressed for an invalid number of times (e.g. greater than the number of available operating modes), the siren emits 5 beeps and is cancelled and the country code is not changed:
set the ignition key to the Key-On position within 10 s of the last press on the rear fog light button.
the siren emits the same number of beeps as the number which indicates the selected operating mode: each beep lasts 1 s and is followed by a 0.5 s pause.
at the end of the beeps, set the key to the Key-Off position: the BC is programmed in the new operating mode.