Change datetime format and language on oracle

SELECT USERENV('language') from dual;

SELECT to_char(sysdate,'dd-mon-yyyy') FROM dual;

ALTER SESSION SET nls_language = 'AMERICAN';

SELECT to_char(sysdate,'dd-mon-yyyy') FROM dual;

ALTER SESSION SET nls_language = 'THAI';

SELECT to_char(sysdate,'dd-mon-yyyy') FROM dual;

http://www.dba-oracle.com/t_nls_date_format_sysdate.htm
http://infolab.stanford.edu/~ullman/fcdb/oracle/or-time.html