Setting the NLS_LANG Environment Variable for Oracle Databases:

On Window:

set NLS_LANG=AMERICAN_AMERICA.UTF8

On Unix (Solaris and Linux, CentOS, Ubuntu etc.)

export NLS_LANG=AMERICAN_AMERICA.UTF8

It would also be advisable to set env variable in your .bash_profile [on start up script]. This is the place where other ORACLE env variables (ORACLE_SID, ORACLE_HOME) are usually set.