"Fifteenth" in Engpsh can be translated as "第十五" in Chinese.
It comes from the word "fifteen," which is the number 15 in Engpsh.
Here are some examples of "fifteenth" in use:
Today is the fifteenth of February.
(今天是二月十五日。)
The fifteenth chapter of the book is the longest.
(书的第十五章是最长的。)
She finished the race in fifteenth place.
(她在比赛中获得第十五名。)