(原) ora-12705 cannot access nls data files or invalid environment specified

今天修改了一个参数 NLS_LANG  导致数据库报错。
ORA-12705: Cannot access NLS data files or invalid environment specified
Cause: Either an attempt was made to issue an ALTER SESSION command with an invalid NLS parameter or value; or the environment variable(s) NLS_LANG, ORA_NLSxx, or ORACLE_HOME was incorrectly specified, therefore the NLS data files cannot be located.
Action: Check the syntax of the ALTER SESSION command and the NLS parameter, correct the syntax and retry the statement, or specify the correct directory path/values in the environment variables.

后来只能又把参数改了回来。如果我想改参数而又不报错，改怎么办呢？