If all you are looking for is a hardness, salt will do that, but since salt can't do everything that sodium lactate does, I wouldn't consider using it as a 1:1 replacement for it.
Some examples of what sodium lactate does that salt does not do:
1) It lends a nice, creamy/oomphy feel to the lather, much the same as milks do.
2) It makes HP batter more smooth and pourable/less lumpy.
3) It helps liquid soap paste dissolve easier during dilution.