In file libraries/plugin_interface.lib.php, lines 27 to 30 are using strtolower but since strtolower does not work in Turkish mb_strtolower should be used. I'm uploading the patch I used.
Hugues, I assigned this to you as you are working on the general transformation of string functions.
Hugues,
I assigned this to you as you are working on the general transformation of string functions.