[request/session] refactor language handling: don't attempt to sync web/repo languages
This decouple thing and make code easier to understand. Previous behaviour was fine
w/ validation error that were translated on the server side, but isn't satisfying for
notification and all. Now:
* translations should be done on the ui side (see previous validation error
refactoring)
* consistent behaviour: each side (web/repo) is responsible to deal with its
request/session of language, no weird interaction between them on this topic
* drop on the way the 'session properties' api, used only for that matter.
-> much simpler/cleaner/saner api
The thing missing being a way for user to tell "My favorite language is xxx but I also
want cw to consider my http language header)