We have the real charset ROM dump! Tonino Bisazza extracted it from a Laser 500 he was repairing.
We discovered there are two additional charsets, GERMAN and FRENCH with their relative special letters.
Apparently the choice of the charset is hardwired (see the circuit diagram lines CGA11 and CGA12) but there is also a pin named GT (graphic table?) coming out the video chip that is involved in the selection of the set (I don't understand how though). I wonder whether there is an undocumented feature that is able to switch the charset via software.