Those 404 pages I was getting because SEF and Url rewritting from config was disabled. I was not aware of this, since SH404SEF was elabled, I thought enbling it from Joomla Config will not required.
But, Since, I enabled both options from Joomla Config, I am getting the language which I selected before the submitting the form
Thank you very very much for your kind support to solve this glitch done by me.
Now, The other one problem I notice is "Gtranslate Dropdown Module" does not keeps selected the new changed language in dropdown.It only shows "Select Language" selected regalrless of choose any langauge.