The extended version does show Mathilda in the shower but there is no nudity by Natalie Portman in this film (She celebrated her 12th birthday the day filming began). The extended scenes are composed mostly of Mathilda training to be a cleaner and helping Leon in his work, including a scene where she shoots a drug dealer with paint balls just before Leon shoots him with bullets.
In another scene Mathilda tries to talk Leon into taking her virginity which he flat refuses to do, but it's clear he's tempted. It is clear Leon falls in love with the girl but he never touches her sexually.

In the original script, Mathilda (aged 13 or 14) and Léon became lovers. Luc Besson reportedly altered the script to remove this aspect of the story (possibly due to pressure from Natalie Portman's parents).
In the original script, Leon accidentally walks in on Mathilda when she's in the shower.

LEON
Mathilda? May I come in?

MATHILDA
Yes.

Leon opens the door. Mathilda is naked and is brushing her
hair. Leon closes back the door without entering.

LEON(embarrassed)
Sorry. I heard "yes", so I got in...

Mathilda opens the door. She's still combing her hair. She's
still naked.

MATHILDA
Yes... You can come in.

Leon is rigid. He takes a towel and deploys it in front of
her.

LEON
Take it, please.

She rolls herself into the towel, without speaking.

https://thescriptsavant.com/movies/Leon_(1994).pdf

---