When I read the EULA of softwares sold in France, they often contain restrictions on reverse engineering. In France, reverse engineering is a protected right for interoperability purpose. If some parts of the EULA are illegal, how can it be a legal binding contract.

EULAs like most standard terms will contain severability clauses which basically say that even if one clause is invalid the remainder still holds...

If France(as is in pretty much everywhere in the EU) any EULA that is accepted by default by opening a product(think MS Windows packaging) is void by default, because courts ruled that opening a package does not constitute entering a legally binding contract,no matter what the packaging says.

Because in France, a judge may very well decide that some part of the contract is illegal but consider that the remaining still holds. That happens all the time.

In the EU pretty much any of the online T&Cs as well as EULAs are completely invalid. They can say literally whatever, if you haven't signed an actual piece of paper it's not going through in any court.