Support a multi-lingual TBB that can switch between localizations

I think for our hardened and/or alpha series, we should just include all of our langpacks in one release, and ask the user for their language. The pref general.useragent.locale exists for this purpose.

The reason we want to do this is to avoid another 2G worth of dist files for the hardened series (which will also need special build options, like 64 bit on Windows and Mac).

This is probably a usability sinkhole, though :/