[quote=@AimeChambers]So let's have it that the King knows the thugs were from Tiadan but the queen of Lutaires doesn't.
[/quote]

The queen sent out scouts to find out.  So you could say the scouts discerned that the thugs were from Tiadan.  Or if the thugs took a route through Hiertania before coming to Lutaires, the scouts could conclude that the thugs were from Hiertania.

Are Hiertanians and Tiadains similar enough in appearance and behavior that Lutairites couldn't tell them apart?

I figured that it'd be more realistic to have differing forms of referring to the place, the people, the adjective.  I know Aime used the -ian suffix for all of them, but I think this would add more cultural flavor.  (I am a bit of a linguaphile.)  This setup would also have Tiadan be culturally closer to Hiertania than to Lutaires.
Hiertania-Hiertanians-Hiertanian
Lutaires-Lutairites-Lutairi
Tiadan-Tiadains-Tiadain