Deprecate the session language detection method

Attached you can find how the UI would change.

I spent three hours trying to fix #815544: Remove the language query string when using session language switch links last night but I could not come out with a working solution yet.

In the meantime I realized that the main problem with the session language provider is it was designed to work with the URL language provider (to be able to convey two language types within the same URL) and not as an alternative to it. With content language negotiation out of core the session language has no real meaning, and using it as an alternative to URL language negotiation can potentially cause duplicates both in Drupal's page cache and in the search engines' indexes (which is bad!).

Given this situation I'd strongly advocate removing session language negotiation from core to have time to rework it more carefully.

By the way, another related bug:

#784832: Conflict with the session language provider while creating a translation

@Damien Tournoud, @Gábor Hojtsy:

It breaks the principle of one URL == always the same content

In D6 we had some big efforts to tie URLs to the exact same content without out of URL swicthes to use different languages

Well, the main idea behind the redesign of the language negotiation system was to achieve two separate behaviors (we used wikipedia as an inspiration):

• For anonymous users keep always intact the concept of same URL == same content: browser language and user language are ignored for anonymous users so this leaves us only session and URL. We tried to implement the session language provider so that for anonynous users it always uses the URL parameter, this way pages differing only for the UI language actually have different URLs. The problem here is the opposite: we have multiple URLs for the same content (i.e. #815544: Remove the language query string when using session language switch links).
• For authenticated users we break the concept of same URL == same content - which actually is already broken because the same page might hold different content depending on which user views it - and we allow different languages for the same URL.

@andypost:

Principle if same content on same URL is useless for content editor! I need native language UI (switchable) while editing any multilingual content

Totally agreed. But if we can't fix the session language provider in core, IMO it's better to remove it and fix it in contrib.

By the way, while testing user language as an alternative to session language for content editors, I found this: #855380: $language_url should be used to lookup the current path alias :(

We can't do this anymore.