State immunity — Jurisdictional immunity — Expropriation — Foreign company — Claim against foreign State for damages — Policy of “Zairianization” operated by State of Zaire — Claim in Belgian courts to recover compensation payable under the law of Zaire — Whether State of Zaire entitled to invoke immunity from jurisdiction — Whether “Zairianization” of foreign-owned companies constituting an act of sovereignty — Whether recourse to procedure established by Zairian Government for recovery of compensation constituting an ordinary commercial act — The law of Belgium