Fix bug in bulk app translation with mismatched form/app languages

There was a bug that manifested itself when the `<translation>` elements of a form had different `lang` attributes than the app, and when the `<translation>` element indicated as the default was not the first `<translation>` element. If these conditions existed, then bulk app translation threw a 500. Fixed by removing an implicit assumption about which `<translation>` in the form is indicated as the default.

[Ticket](http://manage.dimagi.com/default.asp?151635)