code: public String validateFallbackLocale() {
          ULocale loc = ULocale.getDefault();
          if (loc != fallbackLocale) {
              synchronized (this) {
                  if (loc != fallbackLocale) {
                      fallbackLocale = loc;
                      fallbackLocaleName = loc.getBaseName();
                      clearServiceCache();
                  }
              }
          }
          return fallbackLocaleName;
      }
docs: Return the name of the current fallback locale.  If it has changed since this was
  last accessed, the service cache is cleared.