“… it is contrary to international law for one state to adjudicate upon the internal affairs of another state. Where a state or a state official is impleaded, this principle applies as part of the explanation for immunity. Where a state is not directly or indirectly impleaded in the litigation, so that no issue of state immunity as such arises, the English and American courts have none the less, as a matter of judicial restraint, held themselves not competent to entertain litigation that turns on the validity of the public acts of a foreign state, applying what has become known as the act of state doctrine.”