FR: Ability to change language on-the-fly

[pyhedgehog]

Right now language of site can only be configured by:

1. User settings (if user had logged-in).
2. Browser language settings (if user is anonymous).
   In some cases (for public sites and for localizer debug comfort) there should be ability to temporary switch language.

I think there should be either query-string parameter:

http://localhost:3000/page/SomePage?lang=ru
http://localhost:3000/page/SomePage?lang=cn

Another implementation can be cookie that can be set by GET request:

http://localhost:3000/auth/setlanguage?lang=ru&redirect=/page/SomePage
http://localhost:3000/auth/setlanguage?lang=cn&redirect=/page/SomePage

Both cases can be used to create menu entries to switch language (that;s why I insist on GET parameters).

Let's pretend those:

1. who prefer query-string for every page (and mechanics to pass it to every link) vote with
2. who prefer GET-request to /auth/setlanguage (without _csrf parameter) vote with

[pyhedgehog]

Found that there are already exists entrypoint /auth/setlanguage. But it can't be used in menu, due to it's POST request and requires _csrf parameter.
So I've changed request to use this entrypoint instead of /auth/switchlang, and propose voting for users.