Hello All,

I have found the solution of this problem and it works fine as i need. here i put some code. When user switch  language into website, so one controller is call and into that controller i have update my code. Whatever new language is select by user on website at that time i have updated that language into user language field(in backend). and update the context value. so now when user goes to backend, at that time user get the GUI in newly selected language which is selected by user on website.

CODE: 

from odoo.addons.website.controllers.main import Website
from odoo.http import request
class WebsiteExt(Website):
    @http.route()
    def change_lang(self, lang, r='/', **kwargs):
    # this controller call when user switch the language into website
        res = super(WebsiteExt, self).change_lang(lang, r, **kwargs) # super call of the change_lang()
        if not request.env.user._is_public():
            request.env.user.partner_id.lang = lang
            request.env.user.lang = lang
            request.env['ir.http'].webclient_rendering_context() # call this method so the menu and session data will be reload.
        return res