"He" is not a mistranslation. The word in issue is the Hebrew emphatic pronoun "הוּא (hu)". While beginners in the study of Hebrew are often taught that "הוּא (hu)" is the masculine third person pronoun and "היא (hiy)" is the feminine third person pronoun, this distinction occurs "beyond the Pentateuch" (Strong's Hebrew and Greek Dictionaries). In Genesis, "הוּא (hu)" is used for all genders, including the feminine and neuter. Even within the same chapter of Genesis 3:15 the same word "הוּא (hu)" is used to mean "she" in reference to Eve, a woman.
An article published by the Ankerberg Theological Research Institute titled "The Case for Jesus the Messiah - Incredible Prophecies that Prove God Exists/Part 5" makes good points regarding the Messianic prophecy in Genesis 3:15 but it is wrong with respect to the KJV translation. It says:
"And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel." (Genesis 3:15, KJV)
ואיבה אשׁית בינך ובין האשׁה ובין זרעך ובין זרעה הוא ישׁופך ראשׁ ואתה תשׁופנו עקב׃ (Genesis 3:15, Hebrew)
"The King James Version (KJV) has made a mistake in translation here. The translators ignored the third person masculine, singular pronoun found in the text and instead of translating the pronoun "he," they mistranslated the pronoun as "it." But the grammar clearly indicates "he." The KJV wrongly says, "It shall bruise thy head;" the Hebrew says, "He will bruise thy head.""
Apparently the original Hebrew language did not have a separate feminine pronoun. While one could certainly prefer that "הוּא (hu)" be translated as "he" at Genesis 3:15, it is wrong to say that it must be translated as "he". As "הוּא (hu)" could refer to things both animate as well as inanimate, the context of Genesis 3:15 is not so clear as to whether it is the "seed" itself or the "enmity" between the feuding seeds that will bruise the serpent's head. In light of this ambiguity, "it" is an appropriate pronoun as both "enmity" and "seed" could be referred to in the neuter. In any event, both Jewish and Christian interpreters regard the "seed" as the Messiah, who is believed to be male in both religious traditions, so the gender of the seed is not the issue. Arguing that it is "he" that will bruise the serpent's head will not resolve any dispute between Jews and Christians.
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