1. European Convention on State immunity – Article 5
Territorial principle;
Exceptions (e.g. nationality: see Article 5).
In the same sense see UK 1978 Act (UK is a party to the European Convention)

2. Private nature of all contracts of employment = No immunity (Belgian case-law)

3. Distinction according to the nature of the functions performed by the employee
If public law functions then immunity
If merely auxiliary functions no immunity
(Italian Court of Cassation, French Court of Cassation, at least one case in the UK, Canada and European Court of Human Rights)

4. Article 11 of the UN Convention on Jurisdictional immunities
Complex approach: a combination of the above mentioned various criteria