In the comic book canon, SHIELD is ambiguously supernational. In other words, not necessarily a purely American department.
It started out as a "somewhere above the Pentagon, and only slightly below the President, but also able to operate autonomously" division, back in the days when pretty much everything that happened in the Marvel Universe centered around New York in general and Manhattan in particular.
As the MU expanded (it was a Big Thing (TM) when they started a California-based branch of the Avengers) it first became less East Coast centric, and eventually less US-centric. They deliberately keep it vague, but the current idea of SHIELD (in the comics, anyway) is sort of... well, sorta-kinda a UN-ish Department of Defense. It's American run and mostly operated, but features support from allied countries like Britain, Germany and Canada.
As such, it stands to reason that there'd be some non-US aircraft, like an Alphajet and a Harrier, and that they also be recognizable as such.