Use async call to detect user language in Tabzilla, Bug 1018724

We discussed this in our Monday meeting and for now we'd probably like to keep things simple. There may be a time when we do need to request this info somewhere outside of the transbar, but if/when that time comes we could make this request directly to the view if needed.

So for now let's just make this AJAX request *if* the transbar is enabled, then just call `setupTransbar` in the callback. Does this sound ok to you @kyoshino?