locale: Allow language selection during first boot

After the implementation of !1227 (closed), first boot interface is available in English and the only way to change the language is via browser preference change.

Show the language selection icon during first boot wizard. Icon should take the users to language selection page and after selecting the language should bring it back to the first boot page they were on.

Consider during this also for first-setup wait message.

Changes will have to be done in the middlewares. Instead of adding yet another exception condition in the middleware consider implementing a decorator.