Already in June last year, the cour grand-ducale issued a statement saying that Prince Jean and his younger brother Prince Guillaume are willing to appear in court to make their witness statements in the Bommeleeër case. The two younger sons of Grand Duke Jean already appeared in front of the judges at the very beginning of the trial early last year when all called witnesses were asked to do so. Based on article 510 and following of the Code d’instruction criminelle, princes and princesses as well as other dignitaries of the Empire - it was written in 1812... - did not need to appear in front of court.
Now the constitutional court has also ruled that the princes shall testify in the case after being charged by the criminal chamber back in June to determine whether the princes, as members of the Grand Ducal Family, can actually be summoned as witnesses. This might not be of such a great importance in this case as Prince Jean and Prince Guillaume had already agreed to appear in court but will likely also mean that in any future case, a member of the Grand Ducal Family will not be able to refuse to appear in court based on the 1812 law.
As a sidenote, I'm pretty sure that the last paragraph of the Wort article which is the base for this article is faulty. To the best of our combined knowledges, there have never "been claims implicating [...] Prince Guillaume with the bombings". He is merely one of the persons - and the only one of them asked to appear in front of the court, I believe - who can confirm his brother's alibi.