ORA-12705: Cannot access NLS data files or invalid environment specified

[oracle@server bin]$ unset NLS_LANG
[oracle@server bin]$ ./sqlplus /nolog

SQL*Plus: Release 11.2.0.1.0 Production on Tue Feb 3 11:19:03 2015

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SQL> connect /as sysdba
Connected to an idle instance.
SQL> startup
ORACLE instance started.

Total System Global Area 1.0088E+10 bytes
Fixed Size                  2215984 bytes
Variable Size            4831842256 bytes
Database Buffers         5234491392 bytes
Redo Buffers               19640320 bytes
Database mounted.
Database opened.
SQL>