code: public static String toUnicodeLocaleKey(String keyword) {
          String bcpKey = KeyTypeData.toBcpKey(keyword);
          if (bcpKey == null && UnicodeLocaleExtension.isKey(keyword)) {
              // unknown keyword, but syntax is fine..
              bcpKey = AsciiUtil.toLowerString(keyword);
          }
          return bcpKey;
      }
docs: <strong>[icu]</strong> Converts the specified keyword (legacy key, or BCP 47 Unicode locale
  extension key) to the equivalent BCP 47 Unicode locale extension key.
  For example, BCP 47 Unicode locale extension key "co" is returned for
  the input keyword "collation".
  <p>
  When the specified keyword is unknown, but satisfies the BCP syntax,
  then the lower-case version of the input keyword will be returned.
  For example,
  <code>toUnicodeLocaleKey("ZZ")</code> returns "zz".
  
  @param keyword       the input locale keyword (either legacy key
  such as "collation" or BCP 47 Unicode locale extension
  key such as "co").
  @return              the well-formed BCP 47 Unicode locale extension key,
  or null if the specified locale keyword cannot be mapped
  to a well-formed BCP 47 Unicode locale extension key.
  @see #toLegacyKey(String)