Wikipedia is invincible when doing this kind of research. Looks like there are infinite number of variations. Now I know John is originally from Hebrew, and it means "God is gracious".
Familiar variations:

• Johnny
• Johan
• Hans

Didn’t know, but kinda understandable

• Jack
• Yohanna
• Jan/Janes/Janoz
• Jean
• Jon/Joao/Jovan
• Hansel
• Juan/Juanni

Shouldn’t they be completely different names?

• Jack
• Evan
• Giovanni
• Ian
• Sean/Shaun/Shawn

Female version

• Jane/Jana
• Joan/Joanne
• Ioana
• Janice/Janet
• Sinead

What’s this?

• Eoin
• Ion
• Yohani/Yohanes
• Giuan