Record current language for all *Log objects

This addresses #680

Internationalization is a long-term key to our interface, and will be of long-term interest in data mining.  It would be great to keep track of the language for which users are viewing videos, doing exercises, and are logged in for.

I believe that in all cases (future design of video logs), we will want to keep language information separately (see my comments at #680).

The following changes record language for:
* Current language when viewing a video
* Current language when doing an exercise
* Current language when logged in

There are a few other changes as far as potential issues--untested by me, needs review by the author (@dylanjbarth)?