So in the radiant high dawn,
The golden face an arrowhead,
A wake cuts amid the cardinal.

(Okay, this is going some...I'd like to hear what others have to say on it, too.)
Looking at the flag and reading the entire poem, it seems like the description of the flag starts with comparing the blue parts to the wings of an eagle, blue like the sky and the sea; then we pass to the white stripe, the "wake", cutting across the "purpurado cuello" (cardinal's neck") with the yellow face in the middle like an arrowhead. Perhaps another word could be better used in English to express the loftiness of the cardinal's neck...?

Or something like that. ;-)

This is why I usually stick to technical translations!