United Nations Mercenary Convention can be read directly from UN website. The full text of the convention can be downloaded by going to the following link
https://www.icrc.org/ihl.nsf/INTRO/530?OpenDocument
Article 1(1)(c) of International Convention against the Recruitment, Use, Financing and Training of Mercenaries, 4 December 1989 says -
1.A mercenary is any person who:
(c) Is neither a national of a party to the conflict nor a resident of territory controlled by a party to the conflict;
Article 1(2)(c) of the same convention says -
2. A mercenary is also any person who, in any other situation:
(c) Is neither a national nor a resident of the State against which such an act is directed;
"If my mother tongue is shaking the foundations of your State, it probably means that you built your State on my land" - Musa Anter, Kurdish writer, assassinated by the Turkish secret services in 1992