Richard Packham, a former member of the LDS Church, and a long-standing critic thereof, wrote in his article, "A Linguist Looks at Mormonism: Notes on linguistics problems in Mormonism,” wrote the following under the heading of “More King James Mistranslations in the Book of Mormon":
The Book of Mormon preserves some demonstrable mistranslations of the King James Version of the Bible. One notable example is Isaiah 7:14, which in the KJV is translated "a virgin shall conceive, and bear a son, and shall call his name Immanuel." This is copied word for word into the Book of Mormon at 2 Nephi 17:14. The problem is that the Hebrew text has the word 'almah,' which does not mean "virgin," but "young woman": the Hebrew word for "virgin" is 'bethulah,' and most modern Bible translations do not use "virgin" to translate Isaiah 7:14. (Some Christians, including the author of Matthew 1:22-23, view this passage as a prophecy of the birth of Jesus from the virgin Mary, but that ignores the entire context of that chapter: the purpose of the prophecy was to answer King Ahaz' question about the outcome of his upcoming war with Syria and Israel.)
The error can be traced back to the fact that the King James translators relied heavily on the Latin (Vulgate) translation of the Bible by Jerome, from the 4th century A.D. Jerome, in turn, relied on the Greek (Septuagint) translation of the Old Testament. In Greek there is only one word for both meanings ("virgin" and "young woman"), making the Greek translation from Hebrew ambiguous. But why would Nephi be confused? He was (supposedly) in possession of the original Hebrew text, which would have had the word 'almah,' not 'bethulah.' But he mistranslates the passage just as Jerome and the King James translators mistranslated it many centuries later.
It is true that the Book of Mormon speaks of Mary being a “virgin,” both in its quotation of Isa 7:14 in 2 Nephi 17:14, as well as various other texts speaking of the then-future mother of the Messiah (1 Nephi 11:13, 15, 18, 20; Alma 7:10). However, there are problems with the argument from Packham.
Firstly, עַלְמָה (young lady) can and is used interchangeably with בְּתוּלָה (virgin). Notice the description of Rebekah in Gen 24:16, 43 (from the 1985 JPS Tanakh translation):
The maiden was very beautiful, a virgin (בְּתוּלָה) whom no man had known. She went down to the spring, filled her jar, and came up . . . As I stand by the spring of water, let the young woman (עַלְמָה) who comes out to draw and to whom I say, "Please, let me drink a little water from your jar."
Indeed, in most instances, a “young lady” would be expected to be a “virgin,” so it is essentially an issue of semantics.
Furthermore, we see this interchangeability of the terms in other Northwest Semitic languages (same language family as Hebrew).
In an Ugaritic poem recounting the marriage between Nikkal and the lunar goddess, we find the following:
tld btl[t]
[lk]trt lbnt hll [snnt]
hl lmt tld b[n]
The first and third lines can be translated thusly:
The virgin (bethulah) bears
. . .
Behold, the young woman ('almah) shall bear a son.
This text is strikingly similar to Isa 7:14.
It should also be noted that The Testament of Joseph, found in the Testament of the Twelve Patriarchs, a pseudepigraphic text dated from the 2nd to 3rd century B.C. (though some scholars argue it evidences some later Christian interpolations, so caveat lector), speaking of the then-future Messiah, speaks of his mother as being a "virgin."[1]
At the very least, the contention of Packham that the Book of Mormon follows a KJV “error” in 2 Nephi 17:14 is not as clear-cut as they like to make it out to be; indeed, there is good evidence that Isa 7:14 should be understood as a Messianic text about a virgin, not just a young lady, giving birth to the Saviour.[2]
Notes for the above:
[1] See "The Testament of the Twelve Patriarchs,” in The Old Testament Pseudepigrapha (2 vols.; New York, Doubleday: 1983-1985), ed. J.H. Charlesworth, 1:775-828.
[2] While I disagree with some elements of his comments about the Hebrew, see Michael Rydelnik, The Messianic Hope: Is the Hebrew Bible Really Messianic? (Nashville: B&H Publishing Group, 2010).