Pentecost is a Greek numeral, and what it numbers is a Jewish agricultural festival.
Shavuot, the Feast of Weeks, is fixed in Leviticus by counting: from the day after the sabbath of Passover week, count seven complete weeks, and on the fiftieth day bring a new grain offering. Fifty in Greek is pentekoste, and the Greek-speaking Jewish diaspora used that name for the festival. The Christian feast inherits both the name and the count, fifty days after Easter, which is why it moves with Easter and why it always falls on a Sunday.
The event narrated in Acts happens at the festival, in Jerusalem, with the city full of pilgrims from across the diaspora, and the list of nationalities who hear their own languages is doing exactly that work: the audience for the sign is the assembled Jewish world.
There is an old calendrical dispute inside the counting. The instruction says to count from the day after the sabbath, and what "the sabbath" means was contested in the Second Temple period: rabbinic tradition took it as the first festival day of Passover, giving a fixed date, while the Sadducees and the Qumran community took it as the weekly sabbath, giving a date that always falls on a Sunday. The rabbinic reading prevailed in Judaism, which is why Shavuot now has a fixed date, the sixth of Sivan, and does not always fall on a Sunday.
Christian Pentecost, by inheriting the count rather than the date, effectively preserves the other reading.