What is the ruling of Islamic law regarding a man resorting to stealing his wife's private divorce letters, hoping she will be absent from court so he can obtain an administrative divorce, given that she refuses divorce and he is prevented from remarrying in France and Tunisia?

Polygyny is permissible if a man is capable of marriage and just between his wives, and no one is allowed to prevent it. If there is a law that prohibits it, then it is oppressive, and it is permissible to get rid of it in a way that causes no harm, including stealing private messages related to it from the wife. A Muslim should be keen to reside in Muslim countries.