std::setlocale(3)

NAME
std::setlocale - std::setlocale

Synopsis
   Defined in header <clocale>
   char* setlocale( int category, const char* locale );
   The setlocale function installs the specified system locale or its portion as the
   new C locale. The modifications remain in effect and influences the execution of all
   locale-sensitive C library functions until the next call to setlocale. If locale is
   a null pointer, setlocale queries the current C locale without modifying it.

Parameters
   category - locale category identifier, one of the LC_xxx macros. May be 0.
   locale - system-specific locale identifier. Can be "" for the user-preferred
              locale or "C" for the minimal locale

Return value
   Pointer to a narrow null-terminated string identifying the C locale after applying
   the changes, if any, or null pointer on failure.
   A copy of the returned string along with the category used in this call to
   std::setlocale may be used later in the program to restore the locale back to the
   state at the end of this call.

Notes
   During program startup, the equivalent of std::setlocale(LC_ALL, "C"); is executed
   before any user code is run.
   Although the return type is char*, modifying the pointed-to characters is undefined
   behavior.
   Because setlocale modifies global state which affects execution of locale-dependent
   functions, it is undefined behavior to call it from one thread, while another thread
   is executing any of the following functions: std::fprintf, std::isprint,
   std::iswdigit, std::localeconv, std::tolower, std::fscanf, std::ispunct,
   std::iswgraph, std::mblen, std::toupper, std::isalnum, std::isspace, std::iswlower,
   std::mbstowcs, std::towlower, std::isalpha, std::isupper, std::iswprint,
   std::mbtowc, std::towupper, std::isblank, std::iswalnum, std::iswpunct,
   std::setlocale, std::wcscoll, std::iscntrl, std::iswalpha, std::iswspace,
   std::strcoll, std::wcstod, std::isdigit, std::iswblank, std::iswupper,
   std::strerror, std::wcstombs, std::isgraph, std::iswcntrl, std::iswxdigit,
   std::strtod, std::wcsxfrm, std::islower, std::iswctype, std::isxdigit.
   POSIX also defines a locale named "POSIX", which is always accessible and is exactly
   equivalent to the default minimal "C" locale.
   POSIX also specifies that the returned pointer, not just the contents of the
   pointed-to string, may be invalidated by subsequent calls to setlocale.

Example
// Run this code
 #include <clocale>
 #include <cstdio>
 #include <ctime>
 #include <cwchar>
 #include <iterator>
 #include <string>
 int main()
 {
     // Make a "deep copy" of current locale name.
     std::string prev_loc = std::setlocale(LC_ALL, nullptr);
     // The C locale will be UTF-8 enabled English,
     // decimal dot will be German,
     // date and time formatting will be Japanese.
     if (const char* loc = std::setlocale(LC_ALL, "en_US.UTF-8"))
         std::wprintf(L"New LC_ALL locale: %s\n", loc);
     if (const char* loc = std::setlocale(LC_NUMERIC, "de_DE.UTF-8"))
         std::wprintf(L"New LC_NUMERIC locale: %s\n", loc);
     if (const char* loc = std::setlocale(LC_TIME, "ja_JP.UTF-8"))
         std::wprintf(L"New LC_TIME locale: %s\n", loc);
     wchar_t buf[100];
     std::time_t t = std::time(nullptr);
     std::wcsftime(buf, std::size(buf), L"%A %c", std::localtime(&t));
     std::wprintf(L"Number: %.2f\nDate: %Ls\n", 3.14, buf);
     // Restore the previous locale.
     if (const char* loc = std::setlocale(LC_ALL, prev_loc.c_str()))
         std::wprintf(L"Restorred LC_ALL locale: %s\n", loc);
 }

Possible output:
 New LC_ALL locale: en_US.UTF-8
 New LC_NUMERIC locale: de_DE.UTF-8
 New LC_TIME locale: ja_JP.UTF-8
 Number: 3,14
 Date: 日曜日 2022年11月06日 20時40分59秒
 Restorred LC_ALL locale: C

See also
   LC_ALL
   LC_COLLATE
   LC_CTYPE locale categories for std::setlocale
   LC_MONETARY (macro constant)
   LC_NUMERIC
   LC_TIME
   locale set of polymorphic facets that encapsulate cultural differences
               (class)
   C documentation for
   setlocale

External links
   1. List of Windows locale names.
   2. List of Linux locale names.