Add language picker to top of window
Right now, the language of the interface is determined based on the configuration of the browser. Most likely, this is not what we want: i) the user should be able to override manually, and ii) the users needs to be able to share a link to the site in which the language is specified.
Probably, a nice solution would be add a small strip to the top of the first section, which shows flags or something similar.
Needs research and proposal. Consult @rejozenger if in doubt.