Langer was a great player, one of the best halves of his era, like Cronk is today. He was not the best of his era, like Thurston is today, and not one of the all time best, like Johns.
Some are saying that Thurston is approaching Johns like levels, but I don't think he is there yet (and may not get there, just because Johns always made the right play in big matches when it counted, and was a much better tackler), so if you think about the gap between Cronk and Thurston today (not huge, but noticeable), I think that is something similar to the gap between Johns and Langer, though Johns was probably even further ahead of Langer than Thurston is to Cronk.
It is not possible to be sure, as they didn't really overlap in time, and the game went through quite a few changes between the early and late 90s.
All great players and great to watch, just trying to "quantify" the difference between them a bit.