Starting with release 2.1 wxWidgets has support for compiling in Unicode mode on the platforms which support it. Unicode is a standard for character encoding which addresses the shortcomings of the previous, 8 bit standards, by using at least 16 (and possibly 32) bits for encoding each character. This allows to have at least 65536 characters (what is called the BMP, or basic multilingual plane) and possible of them instead of the usual 256 and is sufficient to encode all of the world languages at once. More details about Unicode may be found at www.unicode.org.
As this solution is obviously preferable to the previous ones (think of incompatible encodings for the same language, locale chaos and so on), many modern operating systems support it. The probably first example is Windows NT which uses only Unicode internally since its very first version.
Writing internationalized programs is much easier with Unicode and, as the support for it improves, it should become more and more so. Moreover, in the Windows NT/2000 case, even the program which uses only standard ASCII can profit from using Unicode because they will work more efficiently - there will be no need for the system to convert all strings the program uses to/from Unicode each time a system call is made.