Is the default locale of a program in *nix "posix"?
Looking at the man
7.4 How Programs Set the Locale
A C program inherits its locale environment variables when it starts up. This happens automatically. However, these variables do not automatically control the locale used by the library functions, because ISO C says that all programs start by default in the standard ‘C’ locale. To use the locales specified by the environment, you must call setlocale. Call it as follows:
setlocale (LC_ALL, "");
Emphasis mine