code: public static RelativeDateTimeFormatter getInstance(Locale locale, NumberFormat nf) {
          return getInstance(ULocale.forLocale(locale), nf);
      }
docs: Returns a RelativeDateTimeFormatter for a particular {@link java.util.Locale} that uses a
  particular NumberFormat object.
  
  @param locale the {@link java.util.Locale}
  @param nf the number format object. It is defensively copied to ensure thread-safety
  and immutability of this class.
  @return An instance of RelativeDateTimeFormatter.