sql - Error: Not a Valid Month

The following 2 requests return target results

select sysdate from dual; - 19.08.2015
   select to_char (to_date ('19 .08.2015 ',' dd.mm.yyyy '),' yyyy / mm / dd ') from dual;
- 2015 / 08/19

Question:
Can I ask a month through the letters? If so, in what language to set and how to see the settings?

select to_date ('15 -jan-10 ') from dual;

Returns an error:

SQL Error: ORA-01843: Not a Valid Month
01843. 00000 – “NOT A VALID MONTH”

Answer 1, Authority 100%

You can and through the letter code, but it is more susceptible to the installed language standards

select to_date ('15-yyan-10 ',' dd-mon-yy ') from dual;
select to_date ('15 -jan-10 ',' dd-mon-yy ') from dual;

The NLS_DATE_LANGUAGUA parameter corresponds to the language setting. You can view the settings through the request

select * from nls_session_parameters;

You can also rigidly specify the language

select to_date ('15 -jan-10 ',' dd-mon-yy ',' nls_date_language = american ' ) from dual;

Yes, you can set the letters in the letters, only you need to specify the date format.
For example

select to_date ('15-yyan-10 ',' dd-mon-yy ') from dual;
select to_date ('15 -jan-10 ',' dd-mon-yy ') from dual;

How exactly is the month you can see in the NLS settings
Or execute such a request:

select to_char (sysdate, 'dd-mon-yy') from dual

More information about date formats can be read here