This is the 13564th most frequent Russian word.
национальностей
"Nationalities."
Here, 'национальностей' is used to refer to the attribute of 'different nationalities.' It's plural genitive form indicates diversity.
В нашей школе учатся дети разных национальностей.
In our school, children of different nationalities study.
The word 'национальностей' is applied in the genitive plural to express the variety of nationalities involved in the club.
Их клуб объединяет множество национальностей.
Their club unites many nationalities.
In this context, 'национальностей' is used in the genitive plural to signify belonging to diverse cultural groups.