Saving the user's preferred language in the session causes browser caching problems

[cnorthwood]

If the user's preferred language is saved in the session, rather than as a cookie, and if you are using page caching with Expires: headers, any pages which the user loaded before switching languages will be loaded from the browser cache, in the old language, as opposed to the newly switched to language!

If it was saved in the cookie, the Vary: Cookie header will cause the browser cache to be invalidated when the language (hence cookie) changes, but if the SessionMiddleware is being used, then it is used there. What would be nice is an option to force the locale middleware (and set_language view) to always use cookies, even if the session middleware is loaded. Perhaps even getting rid of the session storage completely - using cookies will stop gotchas like this imo!

[Ramiro Morales]

Duplicate of #12794.