add parameter after index.php as default in codeigniter

i want to add default language code after index.php while loading page.if i change the language, lamguage code should be replaced.for example it i have default language code as en my url like

http://localhost/test/index.php/en

when i change my language as tamil and the the language code should be replaced as tm instead of en.now my url like below

http://localhost/test/index.php/tm

even i have changed my base url like below the language code will not be added automatically with index.php

$config['index_page']                       = 'index.php/en';