I would like to inform you that I have solved the problem with the turkish unicode alphabet.
The problem was not from PL/SQL Developer, but from the settings of my system.
Here are the constraints necessary to make sure the data is populated correctly.
1- In registery HKEY_LOCAL_MACHINE -->...
Hello,
I downloaded pl sql developer trial for testing.
I right clicked on a table and Selected 'Edit Data'
And I typed turkish characters such as ş and ğ and