Pro-Kremlin disinformation narrative claiming that foreigners fighting for Ukraine are mercenaries and should not be protected under the Geneva Conventions, while on the contrary people fighting against Ukraine are volunteers and should be protected if taken prisoners.
Ukraine called on experienced foreign nationals to join its foreign legion, whose members are regular soldiers of the Ukrainian army and not mercenaries.
As for the Genva conventions protection on POW, as Ilya Nuzov summarises it on the blog of the European Journal of International Law, the nationality of the soldier is not an issue:
"Combatants are members of the armed forces of a party to the conflict or volunteers corps incorporated therein. According to the ICRC Commentary to Article 4, the requirement for membership in the armed forces, or the incorporation of the volunteer corps, is a matter of domestic regulation. But the incorporated unit must be a professional fighting force, fulfilling the criteria spelled out in Article 4A(2) of GC III, and be subordinate to the regular army command. The fact that an individual is a national of a third State and not a national of the armed forces in which they are serving is widely considered to be irrelevant when it comes to determining combatant or POW status."
It is not an unusual practice since other countries also offer foreigners the opportunity to join the foreign legion without being mercenaries, as in the case of the French Légion étrangère.
Reuters listed some unclear legal issues in some countries, but the practice is usually legal, as La libre Belgique or Le Figaro explain.
At the same time, according to TASS citing Denis Pushilin's decree No.114, the seperatist "republic" Donetsk Peoples Republic just invited foreigners to join their armed formation.
Read also related cases of disinformation: Kyiv ordered Azov Nazis to shoot surrendering Ukrainian soldiers, US transfered jihadist groups in Eastern Ukraine to prepare a provocation against Russia.