There are currently three ways to unlock planes in other countries:
Complete 1 T4 plane research to unlock a second nation research line.
Complete 2 T5 planes research to unlock a third nation research line.
Complete 3 T6 planes research to unlock a fourth nation research line.
Attention: As noted on some of our social outlets, War Wings will be removed from stores as of the beginning of December. So, please refrain from performing further in-game purchases as your enjoyment of said purchases may be short-lived.