[At least one version of] Oracle apparently dislikes strings being inserted into data fields, no matter what their format. Here's the workaround I used:

For example. If I want to execute statement:

select to_date('11/20/2003 10:21:00 PM', 'MM/DD/RRRR HH:MI:SS AM') from dual;

.. oracle will complain when my nls_language is set to SLOVENIAN.
after I set it to AMERICAN, everything works fine.

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