XF 1.5 UTF-8 Characters on URLs

Greetings everyone. My forum is a Turkish forum and sometimes we use UTF-8 characters (like ö, ü) in URLs. Problem is that characters should be converted to o and u but instead they're converting to oe and ue. I know it's about utf-8.php file and it can be fixed with Custom URL Romanization add-on but I don't want to use any add-on for this case and don't know which section to edit in that utf-8 file. There are lots of ö's and ü's in that file. Can anyone help me in this case?
 
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