Known Organisation to support youth activities with participants of more than one nation.
the German-French Youth Office: https://www.dfjw.org
Czech-Germany: Center for Coordination of Czech-German Youth Exchange: http://www.tandem-org.de/
Western Balkans: Regional Youth Cooperation Office (RYCO) founded by Albania, Bosnia and Herzegovina, Kosovo*, Montenegro, North Macedonia and Serbia: https://www.rycowb.org/