Sovereign immunity — Foreign States — foreign armed forces stationed in Italy under NATO Treaty — National of receiving State employed as fireman — Contract of employment — Termination of — Claim for back pay — Whether foreign. State entitled to jurisdictional immunity — Whether general principles of international law applicable — Prevalence of treaty rules — NATO Status of Forces Agreement, 1951 — Division of civilian personnel into “civilian component” and local workers — Whether local workers employed under private law relationship — The law of Italy