1. Encoding of user_data must be unicode
2. Replace product name rule in CI_Cart:
   
   PHP

   var $product_name_rules = '\.\:\-_ a-zA-ZsçÇöÖşŞıİğĞüÜ0-9';

   1	var $product_name_rules = '\.\:\-_ a-zA-ZsçÇöÖşŞıİğĞüÜ0-9';

https://github.com/bcit-ci/CodeIgniter/pull/2303