Mutation will be disallowed in general, yes. It's a bit more subtle than that, but you get the gist.

In my language, there are strings, which are fixed size and cannot be reallocated, and string builders, which hide the actual string behind another layer of indirection to avoid the problem with mutation.

That sort of pattern would be used as necessary to avoid the problems with mutation in the same thread. And Send/Sync would take care of the rest across threads.