Initially it would go to local courts, that's the fear that has scared the hell out of the Messi family. The hacienda witch hunt was brutal for the family and they see the system is aligned against them - it would be ugly
Now, me, I would fight but Messi is weighing family (his nuclear & extended one), legacy, money, stresses and probably a variety of other concerns - he doesn't want to go down that road when he has another clean option in 9 months (4 if you count a contract with City). Perfectly reasonable too but altogether disheartening from my POV