## It came about ##

This phrase is used here to mark the beginning of a new part of the story. If your language has a way for doing this, you could consider using it here.

## sons of God ##

Translators could add a footnote saying "It is not clear whether this refers to heavenly beings or human beings. In either case they were beings that God created."

## My spirit will not remain in mankind forever ##

"My breath will not remain in people forever, to keep them alive" (UDB)

## for they are flesh ##

They are mortal.

## They will live 120 years ##

This can also be translated as "They will not live more than 120 years" of "They will live only 120 years." Possible meanings are 1) the normal lifespan of people would decrease to 120 years old or 2) in 120 years everyone would die.