According to Catholicism, what is the original intent of the term “virgin” in Lk 1:26-27?

OP: "But it seems that Isaiah 7:14 (following which Evangelist Luke appears to have used the term `virgin') used the original Hebrew word almah, which per se denotes the marital status of the girl, and not her physical status of being uninitiated into sexual life."

Isa 7:14 Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.

As a sign from God, one's marital status (unmarried) would have nothing to do with the sign. There are uncounted unmarried women. The sign specifically is a virgin (re her physical status) will conceive; that is, one who was in fact uninitiated into a sexual life. The virgin, don't stop there, will conceive.

PS From a Catholic Church point of view, the catechism says this.

497 The Gospel accounts understand the virginal conception of Jesus as a divine work that surpasses all human understanding and possibility:148 "That which is conceived in her is of the Holy Spirit", said the angel to Joseph about Mary his fiancee.149 The Church sees here the fulfillment of the divine promise given through the prophet Isaiah: "Behold, a virgin shall conceive and bear a son."150

In other words, the sign is the virginal conception, as stated. It is not a sign otherwise, but a human account. There is no sense of a sign from God in a young woman getting pregnant. Rather, the sign is a virgin shall conceive.

Gary Michuta is an apologist, author and speaker and a member of St. Michael the Archangel Parish in Livonia. He wrote the following found in Detroit Catholic

Few passages in Scripture have been as hotly contested by non-Christians as Matthew 1:23, “Behold, the virgin shall be with child and bear a son, and they shall name him Emmanuel …” Jewish writers and even the so-called “new atheists” accuse St. Matthew of not knowing Hebrew and thereby mistranslating Isaiah 7:14’s Hebrew word almah as “the virgin shall conceive …” instead of “young maiden. ”What these critics fail to notice is that Matthew isn’t translating Isaiah 7:14; he’s copying it from an ancient Greek translation of the Old Testament called the Septuagint. The Septuagint translated almah as “virgin,” not Matthew. Moreover, the Septuagint was a Jewish translation, so it would be a very odd charge indeed to accuse these ancient Jews of not knowing Hebrew. But why does the Hebrew say “young maiden” instead of “virgin?” The answer appears to be that there was no Hebrew word that directly and exclusively referenced virgins. Therefore, another word would be needed to make the point indirectly. Because it was the cultural norm that young unmarried girls were virgins, almah was a good substitute. If this is so, how did the Septuagint know that almah in Isaiah 7:14 specifically meant “virgin,” and not the more general “young maiden?” That’s a question only answered by the translators. However, if one looks at what was going on during the time when Isaiah was translated, we might find a clue. Work on the Septuagint began roughly around 200 B.C. starting with the Torah (the first five books of the Bible) and the rest of the books were translated afterward until around the time of Christ. Something remarkable happened during this period that few people outside of scholarly circles know. God had promised David that his dynasty and kingdom would last forever (1 Samuel 7:16), but through the disobedience of Solomon and subsequent kings, David’s kingdom of twelve tribes became divided into two kingdoms, and both were eventually sent into exile. The 10 northern tribes of Israel never fully returned, and the two tribes of Judah came back, but without a king and under the oppression of foreign powers. After Alexander the Great (332 B.C. ), the Seleucids controlled Judea until the Jews eventually rose up and pushed back against the Greeks. What happened next from the perspective of a pious Jew at the time was nothing short of a miracle. The Seleucid empire began to collapse, and the Jews once again slowly took charge of their own land. By the time the dust settled, almost the entire former territory of David’s kingdom was under Jewish control. God was restoring Israel! But there was one problem: the kings who ruled during this period weren’t Davidic; they were Hasmonean. The promise of restoration was given to David’s son, not to a Hasmonean. How was God going to restart David’s dynasty when there was no king to bestow the promise that his son would sit on the throne?The translators of the Septuagint might have had this question in mind when they translated Isaiah 7, which addresses a dynastic crisis back in Isaiah’s day. The northern kingdom of Israel and Damascus were threatening to invade Judah and replace David’s descendent, King Ahaz, with a non-Davidic puppet king, a certain “son of Tabeel” (Isaiah 7:6). Isaiah tells Ahaz that God will not allow it, and as a sign that the fall of David’s dynasty would not take place, “the almah will conceive and bear a son …”It’s possible that when the Jewish translators looked at Isaiah 7:14, they saw the key to understanding how God would bring back David’s dynastic heir. The remarkable sign wasn’t that a young maiden would have a son — that’s not very momentous — but that a virgin would have a son. Because God is the true king of Israel (1 Samuel 8:7), he could, one day, promise a virgin that her child would take “the throne of David his father” (Luke 1:32-33). The questions raised by Hasmonean kings might have provided the Greek translators of Isaiah a crucial insight into what made Isaiah’s “young maiden” so remarkable: she must be a virgin.