I have gathered from several Hebrew experts that 'almah' is a virtuous young woman, not necessarily a virgin; she can be a newlywed. However, if she's single, her virginity is a gimme; if it is known she has had premarital sex, she cannot be an almah.

OTOH, a 'bethulah' is a virgin, male or female. But a bethulah isn't necessarily a virtuous or righteous person, as an almah is.

I see nothing wrong with calling almah 'virgin' at isaiah 7:14, as it's normal for a married almah to have a son. However, Isaiah tells Ahaz that God Himself shall provide a sign. Therefore, it would be something outta the realm of 'normal'. And for a virgin to give birth was certainly extraordinary!

We don't know what voice inflection Isaiah used while making this prophecy. He coulda easily impressed upon Ahaz that this almah would be a virgin.

Besides that, the translators have the hindsight of the New Testament facts to guide their translation.