This indicates that the NLS_LANG for the Oracle Client is either missing, or specifies a character set that is incompatible with the 'Ş' character.

You can go to Preferences > Oracle > Connections, add a new variable, select NLS_LANG, and select a "language_territory.characterset" value that specifies UTF8 for the character set. You will need to restart PL/SQL Developer before this takes effect.