Set default language per-user

[Raynes]

We should add the default language as a setting for users.

@amcnamara Might want to start thinking about where we'd put a setting like this.

[amcnamara]

This would be best stored in a cookie, presumably. I don't see any good reason to store this server-side, other than continuity across browsers (not a huge deal).

[Raynes]

Sounds good to me. If we store it in a cookie, we don't necessarily have to have a setting. Would it be fair to just keep track of the language the user last selected? Or perhaps the language the user has used most? Either of these would be easy to implement. If it seems reasonable (and I think it is, given that we have the lang parameter), I'd like to avoid having a setting.

[amcnamara]

I'd go with last used, it's the most straightforward.