This module adds a new method for language detection to Drupal 7, it keeps track of the language by means of a cookie, so the language is preserved no matter of the user is logged in or not. It also works for recurring users.
You can specify the name of the cookie (default is language) and the duration of the cookie (default 7 days).
The cookie is set from the moment the module is activated, to make use of the detection you have to enable the cookie detection at admin/config/regional/language/configure and make sure it's after the URL one.
Whenever the language can not be determined by the URL detection, it will use the cookie to set the language.
BTW: This module does the same as language_cookie, the only difference is the name.