Interfaces are only useful if there is reason they won't change otherwise they get in the way. Reason is often policical - this is where another group talks to my code and so the less we have to know about the other the better (not that the code is hidden just that you ignore it) reason could also be it is used everywhere so any change would be months of effort to make - longer if anyone else needs to impment anything.

if change is easy then better to make the change - you never really know all future requiresments so getting it right is impossible.

unless getting it right is possible. if you are implementing a new language you should know enough to get the interface to your strings right on the first try. That is not the normal case. (Strings will soon become impossible to change)