EY! the bug is not in the function 'utf8_decode'. The bug is in the function 'mb_detect_encoding'. If you put a word with a special char at the end like this 'accentué', that will lead to a wrong result (UTF-8) but if you put another char at the end like this 'accentuée' you will get it right. So you should always add a ISO-8859-1 character to your string for this check. My advise is to use a blank space.
I´ve tried it and it works! 

function ISO_convert($array)
{
    $array_temp = array();
     
    foreach($array as $name => $value)
    {
        if(is_array($value))
          $array_temp[(mb_detect_encoding($name." ",'UTF-8,ISO-8859-1') == 'UTF-8' ? utf8_decode($name) : $name )] = ISO_convert($value);
        else
          $array_temp[(mb_detect_encoding($name." ",'UTF-8,ISO-8859-1') == 'UTF-8' ? utf8_decode($name) : $name )] = (mb_detect_encoding($value." ",'UTF-8,ISO-8859-1') == 'UTF-8' ? utf8_decode($value) : $value );
    }

    return $array_temp; 
}