For the record, here is the answer sent by mail:

Alexander Kogtenkov from EiffelSoftware is also suggesting to replace this code:

  create string_array.make_from_special(string.as_string_32.area)
  create natural_16_array.make_filled (0, string_array.lower, string_array.upper)
  from
      i := string_array.lower
  until
      i > string_array.upper
  loop
      integer := string_array.item(i).code
      natural_16_array.put(integer.as_natural_16, i)
          -- forth
      i := i + 1
  end
  a_string := natural_16_array.to_c

with
a_string := {UTF_CONVERTER}.string_32_to_utf_16_0 (string.as_string32)

This code will make sure that a_string is 0-terminated
Unicode code points above 0xFFFF (for examples, for emoji) are correctly translated into UTF-16 surrogate pairs 
it is more efficient than the original

If string is in UTF-32, even simpler (and even more efficient code) is possible:
    a_string := {UTF_CONVERTER}.utf_32_string_to_utf_16_0 (string)

Note that in that case, there is no need to perform conversion and to call as_string_32, because utf_32_string_to_utf_16_0 accepts READABLE_STRING_GENERAL.
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Closing this report.