What you display for the group and decimal separators depends upon your NLS settings. To switch dots and commas, change the NLS_NUMERIC_CHARACTERS:
create table t (
x number
);
insert into t values (1234.56);
alter session set NLS_NUMERIC_CHARACTERS = '.,';
select * from t;
alter session set NLS_NUMERIC_CHARACTERS = ',.';
select * from t;
Note this setting is in the client. So the number is still stored in the same way. Different people in different locations can view it according to their preferences.
For further control over how you display the number, use to_char with the appropriate format mask:
select to_char(x, '999G999D00') sep, to_char(x, '999990D00') no_sep
from t;
SEP NO_SEP
----------- ----------
1.234,56 1234,56
http://docs.oracle.com/database/122/SQLRF/Format-Models.htm#SQLRF00211