The preferred/default language is not always English

Introduce a new variable to configure the preferred/default language (default value 'en').

One might code an application in French and then translate it to English. Then you don't want the "[MISSING]" thing everywhere in debug mode if the preferred language (French in this case) is the current one.

Note: I have a doubt how to name this variable, `preferredLanguage` or `defaultLanguage`?
angular-translate uses [`preferredLanguage`](http://pascalprecht.github.io/angular-translate/docs/en/#/api/pascalprecht.translate.$translate#fallbackLanguage).