There are many dead languages we can't figure out. There are very few Rosetta stones. We have not figured out any animal language. I doubt we'd have any better luck with ET.
Depends upon your E. T., how they appear among us, what they do among us, and how they come here and go away without most people realizing who or what they are. Remember, we are apparently a backwater, fly over country, rural planet without much about it or us that would interest your usual multi-universe, multi-species, mainstream traveler. One day we might join the fray as a minor planet of life among too many too far spread to bother with. Among so many that do not even bother with origins since many probably can't remember theirs and deal only in time honored, now fictional, legends of their pasts.
I have no doubt there is life out there, just as complex and varied as ours, their top rungs just as imaginative, complex, varied and thus chaotic ("infinity of mind") as Mankind is. But that means the tyrant, the savage intelligent raptor as well as the Han Solo sort. What bothers me is the sweet little movie "E. T." or the supposedly ultra-civilized, ultra-powerful (power corrupts, and absolute power corrupts absolutely), Gestapo-like policeman and robocop of "The Day the Earth Stood Still" would prove to be far more insidiously dangerous to us than the outright predators of the "Predator" and "Alien" kind. The species that take their more [independent than not] place among the more civilized, yet prove they are almost too dangerous to mess with will be the species that "live long and prosper," as the saying goes.