Some stuff they say over the microphones is just random, jaw-droppingly random. When I watchedthis film in Italian (in my teen years) I remember the film as being quite confusing and now I totally understand why. You still get what's happening but the nerve of these translators!
I don't know if some of the plot alterations have been requested by Warner (who, we may assume, re-cut the film in this order before presenting it to the dubbers) however I can list some of the main differences, plot-wise:
1) the hologram is a projection of THX's mind, in Italian they call it "transfert di pensiero" which I translated as "transference" (it's a term from psichology also used in paraphsychology). They are very clear about the fact that THX generated this hologram. In the English version I think it was much more vague and if I'm not mistaken, the hologram coming into the real world had nothing to do with THX's mind.
2) there's a lot of talk about "germs of thought" which can infect society. THX is almost a biological hazard on the run.
3) THX at the trial is considered curable and will be eventually put back to work. Even while he's escaping, they tell him over the radio that he can still get his job back
4) in one screen it is said that SEN corrupted THX. While LUH is only accused of sexual violation. The trial is only about that (after they have sex in the white limbo) - reason why the trial sequence was placed afterwards.
It's all very confusing I know, it seems that Warner wanted a slightly different plot. Some pieces are cut away alltogether.
5) minor plots have been deleted. Totally absent from the translation the fact that someone made a mistake starting a mindblock while he was working at something very dangerous. Then control room declare itself not responsable for the mistake. Another is the scene when THX is in the limbo and operators move him through a mind control equipment. This was changed with the operators talking about THX situation and danger he brings to society instead of being an operator teaching to a trainee how to operate the mind-control equipment (from what they say they weren't controlling him at all, making some of Duvall's spasms totally not understandable... part of which were also cut away)
If I can remember any other difference I'll post it here