"twelfth" is the Engpsh way to say "第12" in terms of pronunciation. The word comes from Old Engpsh "twelf" and "þaetel".
Examples:
The twelfth of never.
绝对不会发生的事情
On the twelfth day of Christmas, my true love gave to me, twelve drummers drumming.
在圣诞节的第十二天，我真爱的人给了我十二个鼓手敲鼓
The twelfth chapter of the book.
书的第十二章