box: fix shared lang between connected clients

Fixed the implementation of the box console.
Before this fix, result of `\set language` is shared between clients
via `console.connect`, despite the fact that clients have different
`box.session.id`. Now the parameter of the selected language is stored
by each client in his own `box.session.storage`.

Fixes #8817

NO_DOC=bugfix

(cherry picked from commit e4fda4b7)