Sovereign immunity — Foreign States and agencies — Central bank of foreign State — Action for repayment under loan agreement between commercial banks — Requirement of repayment through central bank pursuant to foreign exchange legislation — Attachment of bank deposits of central bank — Whether central bank entitled to immunity from jurisdiction and enforcement — Whether fact that central bank endowed with independent legal personality decisive — European Convention on State Immunity 1972, Article 4 and 27 — Whether declaratory of existing international law — Acts iure imperii and iure gestionis — Whether original nature of relationship decisive — The law of Switzerland