Hey I can answer that. Apparently Matthew mistranslated it too.
Matthew 1:22 Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, 1:23 Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
BigMack
Yes, Matthew did mistranslate it. Actually, he based his quote on the Greek translation of the Hebrew, which introduced an ambiguity which is not present in the original. "Virgin" is incorrect. Check just about any new bible today, and you will see it properly rendered as "young woman" or some variation of this.
BTW, Matthew also says "and they shall call his name Emmanuel". Please point me to the passage in scripture where this 'prophecy' is fulfilled, and Jesus IS ACTUALLY CALLED "Emmanuel".