Need language dropdown (in debug mode?)

This issue has been migrated from mozilla/webmaker-android#2060.

It was originally written by secretrobotron on Tue Jun 02 2015 and had the following description:

Language will be picked up automatically from chosen device locale, but we should have a dropdown (preferably in a debug menu) that lets you override the selected language. This is helpful for debugging and for localizers that want to flip back and forth between languages.

Assigning **@**thisandagain because he's working on a debug menu.