try enumerating:

for i, language in enumerate(langlist):
  print(greetlist[i])

anyway, it would be better that you generate a dictionary like {'language' : 'greet'}, so you can call the language and get the greet directly

• You're using .index() without any argument - that's an error.
• You're ignoring the argument of the function - language.
• You are always returning something in the first iteration of the loop, so this won't work. You don't really need a loop here at all.