I found 3 problems related to language switching :
1. when doing switching several times on the same pages, lcMessages=xx is append to the url several times
2. when lcMessages=xx is in url, display is completely broken (civicrm css ?)
3. switching language is changing language default in civicrm/admin/setting/localization?reset=1
I suspect all those issues are linked but otherwise, i suppose we should open one issue for each.
Configuration: Drupal 7 with CiviCRM 4.3.beta1 in multilingual mode (fr_CA + en_CA)