The definition of mercenary is being discussed in another thread.
Use of Private Military Contractors & Mercenaries in the eye of International Law (by Amaa'n) The presence of private military and security companies (PMSCs) in armed conflicts has traditionally drawn scant attention. In some ways this is surprising; as such, reliance on private entrepreneurs...
defence.pk
The definition is actually a lot more trickier than just whether or not they are foreigner or they get pay to do the fighting, it is rather related to whether on the combatant role and whether or not they are non-state actors.
In this sense, any state actor, which usually mean state militaria, state paramilitary and group that represent the state, like Law Enforcement, Civil Defence and so on, are State Actor. Non State Actor is group that was not sponsored by the state, ie private security, volunteer group, partisan, insurgent are all non-state actor.
On the other hand, it does not need to be local resident. State Sponsored force can have foreigner in their rank, given if they are treated the same way as other local force member, and was offered a pathway to residency, then it's okay, example such as British Commonwealth Soldier serving with the British Military, or French Foreign Legion serving with French Army, and US Green Card holder serving US Military.
On the other hand, to be called a mercenary, you need to be engaged in combatant role, a red cross volunteer medic, for example, is not a mercenary. a non-combatant role still has inherited right to self-defence, or defence of other in need, so even a non-combatant engage in combat, may still not be class as mercenary.
In this case, Ukraine International Legion, serving under the banner of Ukranian National Guard, and is paid the same as normal Ukrainian soldier in rank, and will offer pathway to citizenship, is not considered mercenary at war. However, as a private security firm, Wagner operator is not a state sponsored militia, and the action of using them as combat troop is in violate the law of combatant via Geneva Convention. Even if these people are Russian, they would still be considered Mercenary in Ukraine.
