In many cases establishing their nationalities is just "This is Phillipos! He's Greek" (may have the Greek dude's name wrong), so it didn't take up that much page time. The twists and turns of the plot were the big page hog, which is natural for a thriller, but there's only so much time left to spend on the Tragedy of Nationalism...

genarti's theory is that the core problem was the genre demand for a Plucky YA Heroine, which may explain why Stella feels somewhat generic, which has a ripple effect on the rest of the cast. I don't read enough YA nowadays to know if that is indeed a necessity of the genre, though. (And you'd think that Elizabeth Wein, author of smash hit Code Name Verity, would get some leeway! But who can say.)