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

A couple things:

1. `_resp()` enforces using the CDN, which we don't want for this. I think that's possibly why the first CDN URL removal you did failed to work. 
2. A template for this seems like overkill. Should be able to just do `'_({"languages":%s});' % json.dumps(l10n_utils.get_accept_languages(request))`. Also using `json.dumps` seems better than the replace in the template.