At the end of the day i've found my solution
I've uninstalled Connector/ODBC 5.2.5 and installed 5.1.12, now i'm good with Turkish characters.
Yes, you found the right solution! MySQL has oddly made separate ODBC drivers for ASCII vs. Unicode support, and the naming convention of the new drivers broke the way Tableau attempts to locate the best driver match for connecting to the database. A future Tableau release should address this problem with the driver naming, but in the meantime you should find very little different between the MySQL v5.1 and v5.2 drivers when used with other applications.