There is two things you'd need to do:
1. Configute the "Limit mode" setting of the MEGA plugin according to
this help article.
2. You'd need to develop some kind of script that will auhto change your NordVPN location.
Maybe they even allow it to use a different location on each start so simply restarting NordVPN would be enough.
You'd then simply need to setup that script as a reconnect script in JDownloader.
While I do not have a solution for you, you might be able to find one in our forums or create a script on your own.