The following information has helped others with this error in the past:
Follow the steps in one, or both, of these Burleson Consulting Knowledge Base articles below. Try step 1, then step 2 if the first does not work.
Burleson Consulting: Resolving the ORA-12705 error:
a. Excerpt: "It's likely a missing NLS_LANG setting in your Windows PC client registry. The NLS_LANG variable is in the Home0 directory in the Windows registry is not the only NLS_LANG, and you need to locate the Oracle directory in the registry and find NLS_LANG. This variable may be set to "NA", and a change to AMERICAN_AMERICA.WE8ISO8859P15 may fix your connectivity issue."
Burleson Consulting: Resolving ORA-12705: Cannot access NLS data files or invalid environment specified:
a. Excerpt: "For ORA-12705 errors caused by invalid NLS_LANG settings, you need to verify that it is 'unset' at the system-level. [In Windows, sic] The NLS_LANG must be unset in the Windows registry (re-named is best). Look for the NLS_LANG subkey in the registry at \HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE, and rename it."
- Burleson Consulting Resolving the ORA-12705 error: http://www.dba-oracle.com/t_ora_12705_error.htm
- Burleson Consulting Resolving the ORA-12705: Cannot acces NLS data file or invalid environment specified: http://www.dba-oracle.com/t_ora_12705_resolution.htm
Hope this helps!