C++ (and do a lesser extent C) lose support because of their extremely poor support for utf8. And the absurd part of it is that they could easily do a good job. Utf8 is just a byte array with various routines to interpret the code. Glibc does a reasonable job for a C library...not ideal, but reasonable.
All the array needs is a way to address a chunk by character # rather than by byte #, a way to copy of a character or a slice of chars, and a way to determine the general character classification of any character. Also a few methods: first(), last(), hasnext(), hasprior(), next() and prior(). And these all "sort of" exist, except getting the general character classification. (Do note that these functions need to operate on utf-8 characters rather than on bytes.) But several different ways of doing this are already known. Vala, e.g., handles it without difficulty, and is able to emit C code (using Glibc libraries).
So it's not a programming difficulty that's holding things up. It's the standards bodies...or, perhaps, some members of them.
But I've looked at C++11, and it is not a satisfactory answer. Vala has a good answer. D (Digital Mars D) has a different good answer. Even Python3 has a pretty good answer. (I don't like that in Python you can't determine memory allocation within the string.) Also Racket, etc. But C++ doesn't.