Ah, C'tis always does that. Reverend, get to it!
I think, if Auld Alliance isn't generally referred to as Auld within the target audience it would be lost to others than me. The first thing that comes to mind is invariably Auld Lang Syne. If you named your prophet Auld Alliance, you might get a funny look or two but I don't think many would consider it a breach. If going by the scout route there's little need for code... In theory someone could spy the battle but that's about it. That, too, could be avoided by using assassins instead
There's also the problem of getting the recipient to check the name of the scout, come to think of it...
Finnish is one of the most difficult languages in the world. It's part of the Fenno-Ugrian family, whereas most European languages are Germanic or Romanic, which explains in part why it's so freaky. Oh, and y'all: Tervetuloa Suomeen!