State immunity — Jurisdictional immunity — Chauffeur employed by embassy of foreign State — Contract of employment — Dismissal — Claim for damages for unjustifiable dismissal — Whether foreign State entitled to jurisdictional immunity — Whether fact that employee is a foreign national is material — Relevance of European Convention on State Immunity, 1972, Article 5(1) — Whether applicable as codification of customary international law even though defendant State not a party — The law of Belgium