Oracle 11g Release 2

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select value
from nls_session_parameters
where parameter = 'NLS_NUMERIC_CHARACTERS';

VALUE

.,

    select to_char(231.12, '99999D99')  from dual;

TO_CHAR(231.12,'99999D99')

   231.12

    select to_char(231.12, '99999D99','NLS_NUMERIC_CHARACTERS='',.''')  from dual;

TO_CHAR(231.12,'99999D99','NLS_NUMERIC_CHARACTERS='',.''')

   231,12