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1) Download and install an 11g client. The latest version of SQL Developer requires an 11g client if you are using the thick/OCI driver.
2) Don't use the OCI driver. Tools > Preferences > Database > Advanced. UNcheck Use OCI/Thick Driver
I had the same error. What worked for me was going into Registry Editor and delete the NLS_LANG key, found under HKEY_LOCAL_MACHINE > SOFTWARE > ORACLE_HOME. Then closed and reopened SQL Developer and problem was gone.