NLS issues

11g
I installed the software with only English language but I see error messages with my local language.
How is that possible?
--- solution ---
alter session set nls_language='AMERICAN';

Consider altering init.ora
Consider setting nls_territory='AMERICA'

Multiple language installation
When Oracle software was installed with multi language support, 
NLS_LANG environment variable is the effective reference.

Alter session examples
ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YY HH24:MI:SS';