If a 10 year old was violated by two adults around 18 years old then can the victim sue 23 years later?
What is the process?
Would it, for example, make any difference if the victim is from, say, Morocco now living in France and one of the alleged offenders is in Italy?
That is happening all the time. Accusations of historic child abuse are often made, often investigated, and often punished. Where the victim is from doesn't make a difference in most countries. Most crimes are prosecuted in the country where they happen. Some crimes, especially sexual abuse of children, will often also be prosecuted in the country that the offender is a citizen of, especially if the country where it happened is hesitant to prosecute.
In your example, if the abuse happened in Switzerland, Switzerland will try to get the offender extradited from Italy, and will ask witnesses to come to Switzerland to make statements in court. Italy might prosecute the alleged offender if he is Italian, they might even prosecute an Italian resident, but I don't know their laws.
You should differentiate between civil and criminal law.
In civil law, you seek restoration of your damages. There, you sue the people that caused you damages to pay you back. It is a "private matter" between the two of you, that is settled by a court.
In criminal law, an individual broke a law, and the state prosecutes him. In theory it does not matter if the victim wants it or even agrees with it (although if it does not agree, it usually it is not prosecuted because it becomes an uphill battle). It is a matter between the state and the accused.
So, the cost for a criminal proceeding could be zero. You go to a police station, denounce the facts, and allow the criminal justice system to follow its path. Of course, it might be that you may want to spend money to do some research (e.g. searching for possible witnesses), or to prepare to give testimony in trial, but that would be optional and theoretically unrelated to the criminal investigation. Of course, the more credible evidence you give to begin your accusation, the more likely it is that the police/attorneys will take it seriously and follow up with it.
And of course, there is the statute of limitations issue, but the tendency for crimes against children is to make them count only after the victim is of adult age.
Most offenses have an attached statute of limitations (French: prescription) which limits how much later after the act one can be sued for past actions.
In France, this can range from a year (for "contraventions", e.g. minor offenses) to unlimited (for crimes against humanity), with most cases at 6 years for misdemeanours (délits) or 20 years for crimes.
For some misdemeanours against children, the statute of limitations is extended to 10 or 20 years, and only starts at the child's majority.
Also, the statute of limitations has changed many times over the the last 30 years, and depending on the date of the offense, the type of offense, who committed it (a parent, a teacher, etc.), the associated statute of limitations may or may not have been extended. The exact rules are complex, there's a calculator here. Based on the info you gave, it would seem that in your case the statue of limitation ends when you turn either 38 or 48, depending on the details.
Note for instance that an "agression sexuelle" (a misdemeanour) will not have the same statute of limitations as a "viol" (a crime). Part of the procedure will be to evaluate whether the offense was one or the other, as that has a big impact on how it is handled (statute of limitations, type of trial...).
There are several ways you can start the process:
The fact that multiple countries are involved does complicate things, as there may be territoriality issues based on where the offenses happened, and proceedings against people in different countries are often more complex (it may make sense to file the complaint in the country where the offender lives, otherwise France may need to request extradition of the offender, which is a process which can take ages).
I strongly recommend you talk to a lawyer first, or contact one of helplines which specialise in these cases. I'm not sure which ones are the best suited, though, and that may depend on your specific situation. This may make the difference between your complaint being dismissed (classée sans suite) for some reason or other and an actual trial, damages, etc.
