Windows: replace deprecated OS calls in SystemStats::getDisplayLanguage()

The previously used getUserDefaultUILanguage() was based on the deprecated
LCID/LANGID concept. It had an increasing number of corner cases as not
every locale had associated LCIDs. The new solution is based on locale
names (string based) and will return reasonable names in more situations.

modules/juce_core/native/juce_win32_SystemStats.cpp

@@ -554,20 +554,36 @@ String SystemStats::getUserRegion()       { return getLocaleValue (LOCALE_USER_D
 String SystemStats::getDisplayLanguage()
 {
     DynamicLibrary dll ("kernel32.dll");
-    JUCE_LOAD_WINAPI_FUNCTION (dll, GetUserDefaultUILanguage, getUserDefaultUILanguage, LANGID, (void))
+    JUCE_LOAD_WINAPI_FUNCTION (dll,
+                               GetUserPreferredUILanguages,
+                               getUserPreferredUILanguages,
+                               BOOL,
+                               (DWORD, PULONG, PZZWSTR, PULONG))
 
-    if (getUserDefaultUILanguage == nullptr)
-        return "en";
+    constexpr auto defaultResult = "en";
 
-    auto langID = MAKELCID (getUserDefaultUILanguage(), SORT_DEFAULT);
+    if (getUserPreferredUILanguages == nullptr)
+        return defaultResult;
 
-    auto mainLang = getLocaleValue (langID, LOCALE_SISO639LANGNAME, "en");
-    auto region   = getLocaleValue (langID, LOCALE_SISO3166CTRYNAME, nullptr);
+    ULONG numLanguages = 0;
+    ULONG numCharsInLanguagesBuffer = 0;
 
-    if (region.isNotEmpty())
-        mainLang << '-' << region;
+    // Retrieving the necessary buffer size for storing the list of languages
+    if (! getUserPreferredUILanguages (MUI_LANGUAGE_NAME, &numLanguages, nullptr, &numCharsInLanguagesBuffer))
+        return defaultResult;
 
-    return mainLang;
+    std::vector<WCHAR> languagesBuffer (numCharsInLanguagesBuffer);
+    const auto success = getUserPreferredUILanguages (MUI_LANGUAGE_NAME,
+                                                      &numLanguages,
+                                                      languagesBuffer.data(),
+                                                      &numCharsInLanguagesBuffer);
+
+    if (! success || numLanguages == 0)
+        return defaultResult;
+
+    // The buffer contains a zero delimited list of languages, the first being
+    // the currently displayed language.
+    return languagesBuffer.data();
 }
 
 } // namespace juce