Unicode.inc locale change breaks PHP charset conversion functions

In line 42-43 of includes/unicode.php, we have the following locale change:

// Set the standard C locale to ensure consistent, ASCII-only string handling.
  setlocale(LC_CTYPE, 'C');

This causes charset conversion of strings using iconv() or mb_set_encoding() to fail.

For example, the following code outputs 'St?phanie,D H?rouville' instead of the expected 'Stephanie,D Herouville':

$string = "Stéphanie,D Hérouville";
$output = iconv("UTF-8", 'ASCII//TRANSLIT', $string);
print $output; 

The workaround is to explicitly set the locale to a UTF8 locale prior to making a call to iconv() or mb_set_encoding(), and switch it back to 'C' immediately after.

I don't like how this is handled. I set my servers up with a certain locale, and that's what I expect it to be when I am coding.

We need to do one of two things:

1. Document clearly that Drupal changes the locale at the beginning of each request.
2. Change the functions which require the 'C' locale for ASCII-type string handling to set it themselves, and switch back after.

The big issue here is DX - I spent a couple hours trying different things and googling to find a solution to this. In hindsight, it's pretty clear, but for a less than perfect developer, this could be a cause of confusion / time waster.