for turkish language I => i and i => I conversion is a problem. It must be I => ı and i => İ so my simple solution is
public function title_case_turkish($str){
$str = str_replace("i", "İ", $str);
$str = str_replace("I", "ı", $str);
$str = mb_convert_case($str, MB_CASE_TITLE,"UTF-8");
return $str;
}