Localization settings can be set on Database level, Instance level (imagine geographically despersed cluster) , Session level
Environmental variable NLS_LANG allows to override Language settings on a session level.
For example you have an application deployment in Italy, obviously you’re expecting data to come in an Italian/European format here is the settings for your Unix box:
To find out your setting use this handy query:
select * from nls_database_parameters;
Will you quite a list of information on your settings, such as:
NLS_CURRENCY <euro sign here>
More information can be found at Oracle: