switching language requires users to login again
Check out the issue here for i18n.

It seems that the Single Sign-On module should help you out.


  This module enables 'single sign on' between multiple web sites that
  share common user tables (see http://drupal.org/node/2622#advanced).
  
  Single Sign-On (SSO) means that if you are logged onto the master
  site, you will automatically be logged into any slave site that you
  visit.
  
  SSO needs at least two Drupal sites running in order to use it (even
  if you are using Domain Access module!). One is called Controller,
  second (third and others) are called Clients. See README for more
  information about setting this up.


I'd think that as the issue in the queue states the cookies are stored per language.  This should be amendable in settings.php.