"This is because." Zechariah and the "many" will rejoice because John will be "great in the sight of the Lord." The rest of verse 15 tells how God wants John to live.
The words "joy" and "gladness" mean the same thing and are used to emphasize how great the joy will be. AT: "you will have great joy" or "you will be very glad" (See: rc://en/ta/man/translate/figs-doublet)
"because of his birth"
"he will be a very important person for the Lord" or "God will consider him to be very important"
This can be stated in active form. AT: "the Holy Spirit will empower him" or "the Holy Spirit will guide him" Make sure it does not sound similar to what an evil spirit might do to a person. (See: rc://en/ta/man/translate/figs-activepassive)
"even while he is in his mother's womb" or "even before he is born"