According to Wikipedia's Latitude page, longitude and latitude (along with elevation) are the geographic coordinate system standard in ISO (International Organization for Standardization) 19111. But the closest it comes to saying which order to put the coordinates in is "the order of coordinates is identical to the order of the axes of the coordinate system." Since the coordinate system is x, y, z (east/west, north/south, and up/down) I'm thinking that the standard says longitude first, latitude second, and altitude third.