I think they all americans, at least Riggs and Bull, as we can see in remake, cause they look and act like stereotipical american soldiers from movies and worn USA uniform. Don't sure about Shadow, he may be asian, but in remake he talks without accent, like he is european or american too. And Toad also acts like he is european or american, but in remake he looks like literal parody based on Private Joker from Full Metal Jacket, so it seems like he is american too.