According to wikipedia, "hey" originates from a "natural expression":
"From Middle English hey, hei, also without h- in ey, from Old English *hē, ēa (interjection), attested as first element in hēlā, ēalā (“O!, alas!, oh!, lo!”). Cognate with Dutch hé, hei (“hi, hey”), German hei (“hey, wow”), Danish and Swedish hej (“hello, hey”), Faroese hey (“hey, hello”), Old Norse, Icelandic and Norwegian hei (“hey”), Romanian hei, Russian эй (ej, “hey”); see heigh.
Probably a natural expression, as may be inferred from its presence with similar meaning in many other unrelated languages: for example, Burmese ဟေး (he, Finnish hei, Unami hè, and Mandarin 哎 (āi), and various sound-alikes as Ancient Greek εἴα (eía) and Latin eia, eho, Sanskrit हे (he). See also hello. "
From what I have noticed it exists in some unrelated languages but tends to be especially common in the Indo-European languages.
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