Language Selection

Fixes #144. This adds a language selection dropdown to the footer which allows the user to change their language which will result in changing the site content. This will also make it clear that #160 is not an issue.
On dev I currently see `site.js` and `nav-main-resp.js` compressed into [`a9db335ea93f.js`][0] which does not contain this event binding/handler:
```...
$(document).ready(function(){$('#sidebar').focusin(function(e){$('#sidebar').toggleClass('child-focus');}).focusout(function(e){$('#sidebar').toggleClass('child-focus');});$('.messages').on('click','.close',function(){$(this).parent('.alert').fadeOut(500,function(){$(this).remove();});});$('meta[name=_download]').each(function(i,elem){var path=$(elem).attr('content');if(path){$('<iframe>',{src:path}).hide().appendTo($('body'));}});});
...
```

It doesn't look like it's picked up the change to `site.js` though I'm not sure why. 

[0]: https://mobilepartners.allizom.org/static/CACHE/js/a9db335ea93f.js