State immunity — Jurisdictional immunity — Foreign government agencies — Commercial activity exception — Distinction between sovereign and commercial acts — Distinction based upon nature rather than purpose of act — Relevance of main rather than subsidiary terms of the contract — State agency for the promotion of Spanish exports to the United States — Contract of employment requiring employee to market Spanish wines in the United States — Provisions allowing employee option to make contributions to Spanish social security system — Whether entering into employment contract commercial act — United States Foreign Sovereign Immunities Act 1976 — The law of the United States