The C++ language does not dictate any character-set, some compilers may support the use of UTF-8, or even UTF-16. However there is no certainty that anything beyond simple ANSI/ASCII characters will be provided.
Thus all international language support is implementation defined, reliant on what platform, operating system, and compiler you may be using.
Several third party libraries (such as the International Unicode Committee Library) that can be used to extend the international support of the platform.