--- a/intl/locale/LocaleService.cpp
+++ b/intl/locale/LocaleService.cpp
@@ -215,16 +215,28 @@ LocaleService::GetAppLocalesAsBCP47(nsTA
   for (uint32_t i = 0; i < mAppLocales.Length(); i++) {
     nsAutoCString locale(mAppLocales[i]);
     SanitizeForBCP47(locale);
     aRetVal.AppendElement(locale);
   }
 }
 
 void
+LocaleService::GetRegionalPrefsLocales(nsTArray<nsCString>& aRetVal)
+{
+  bool useOSLocales = Preferences::GetBool("intl.regional_prefs.use_os_locales", false);
+
+  if (useOSLocales && OSPreferences::GetInstance()->GetSystemLocales(aRetVal)) {
+    return;
+  }
+
+  GetAppLocalesAsBCP47(aRetVal);
+}
+
+void
 LocaleService::AssignAppLocales(const nsTArray<nsCString>& aAppLocales)
 {
   MOZ_ASSERT(!mIsServer, "This should only be called for LocaleService in client mode.");
 
   mAppLocales = aAppLocales;
   nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
   if (obs) {
     obs->NotifyObservers(nullptr, "intl:app-locales-changed", nullptr);
@@ -607,16 +619,33 @@ LocaleService::GetAppLocaleAsBCP47(nsACS
     NegotiateAppLocales(mAppLocales);
   }
   aRetVal = mAppLocales[0];
 
   SanitizeForBCP47(aRetVal);
   return NS_OK;
 }
 
+NS_IMETHODIMP
+LocaleService::GetRegionalPrefsLocales(uint32_t* aCount, char*** aOutArray)
+{
+  AutoTArray<nsCString,10> rgLocales;
+
+  GetRegionalPrefsLocales(rgLocales);
+
+  *aCount = rgLocales.Length();
+  *aOutArray = static_cast<char**>(moz_xmalloc(*aCount * sizeof(char*)));
+
+  for (uint32_t i = 0; i < *aCount; i++) {
+    (*aOutArray)[i] = moz_xstrdup(rgLocales[i].get());
+  }
+
+  return NS_OK;
+}
+
 static LocaleService::LangNegStrategy
 ToLangNegStrategy(int32_t aStrategy)
 {
   switch (aStrategy) {
     case 1:
       return LocaleService::LangNegStrategy::Matching;
     case 2:
       return LocaleService::LangNegStrategy::Lookup;

--- a/intl/locale/LocaleService.h
+++ b/intl/locale/LocaleService.h
@@ -121,16 +121,36 @@ public:
    *   nsTArray<nsCString> appLocales;
    *   LocaleService::GetInstance()->GetAppLocalesAsLangTags(appLocales);
    *
    * (See mozILocaleService.idl for a JS-callable version of this.)
    */
   void GetAppLocalesAsLangTags(nsTArray<nsCString>& aRetVal);
   void GetAppLocalesAsBCP47(nsTArray<nsCString>& aRetVal);
 
+
+  /**
+   * Returns a list of locales to use for any regional specific operations
+   * like date formatting, calendars, unit formatting etc.
+   *
+   * The result is a ordered list of valid locale IDs and it should be
+   * used for all APIs that accept list of locales, like ECMA402 and L10n APIs.
+   *
+   * This API always returns at least one locale.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   *
+   * Usage:
+   *   nsTArray<nsCString> rgLocales;
+   *   LocaleService::GetInstance()->GetRegionalPrefsLocales(rgLocales);
+   *
+   * (See mozILocaleService.idl for a JS-callable version of this.)
+   */
+  void GetRegionalPrefsLocales(nsTArray<nsCString>& aRetVal);
+
   /**
    * This method should only be called in the client mode.
    *
    * It replaces all the language negotiation and is supposed to be called
    * in order to bring the client LocaleService in sync with the server
    * LocaleService.
    *
    * Currently, it's called by the IPC code.

--- a/intl/locale/mozILocaleService.idl
+++ b/intl/locale/mozILocaleService.idl
@@ -78,16 +78,32 @@ interface mozILocaleService : nsISupport
    * (See LocaleService.h for a more C++-friendly version of this.)
    */
   void getAppLocalesAsLangTags([optional] out unsigned long aCount,
                                [retval, array, size_is(aCount)] out string aLocales);
   void getAppLocalesAsBCP47([optional] out unsigned long aCount,
                             [retval, array, size_is(aCount)] out string aLocales);
 
   /**
+   * Returns a list of locales to use for any regional specific operations
+   * like date formatting, calendars, unit formatting etc.
+   *
+   * The result is a ordered list of valid locale IDs and it should be
+   * used for all APIs that accept list of locales, like ECMA402 and L10n APIs.
+   *
+   * This API always returns at least one locale.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   *
+   * (See LocaleService.h for a more C++-friendly version of this.)
+   */
+  void getRegionalPrefsLocales([optional] out unsigned long aCount,
+                        [retval, array, size_is(aCount)] out string aOutArray);
+
+  /**
    * Negotiates the best locales out of a ordered list of requested locales and
    * a list of available locales.
    *
    * Internally it uses the following naming scheme:
    *
    *  Requested - locales requested by the user
    *  Available - locales for which the data is available
    *  Supported - locales negotiated by the algorithm

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -2187,16 +2187,20 @@ pref("converter.html2txt.always_include_
 pref("intl.accept_languages",               "chrome://global/locale/intl.properties");
 pref("intl.menuitems.alwaysappendaccesskeys","chrome://global/locale/intl.properties");
 pref("intl.menuitems.insertseparatorbeforeaccesskeys","chrome://global/locale/intl.properties");
 pref("intl.charset.detector",               "chrome://global/locale/intl.properties");
 pref("intl.charset.fallback.override",      "");
 pref("intl.charset.fallback.tld",           true);
 pref("intl.ellipsis",                       "chrome://global-platform/locale/intl.properties");
 pref("intl.locale.matchOS",                 false);
+// this pref allows user to request that all internationalization formatters
+// like date/time formatting, unit formatting, calendars etc. should use
+// OS locale set instead of the app locale set.
+pref("intl.regional_prefs.use_os_locales",  false);
 // fallback charset list for Unicode conversion (converting from Unicode)
 // currently used for mail send only to handle symbol characters (e.g Euro, trademark, smartquotes)
 // for ISO-8859-1
 pref("intl.fallbackCharsetList.ISO-8859-1", "windows-1252");
 pref("font.language.group",                 "chrome://global/locale/intl.properties");
 
 // Android-specific pref to use key-events-only mode for IME-unaware webapps.
 #ifdef MOZ_WIDGET_ANDROID