Provide english fallback via index.js rather than via the {lang}.json files

[mcwhittemore]

Currently when new strings are added to en.json we have to add copies of these english values to the other {lang}.json files. This is so something is provided in the new case before even if a translation is not provided.

Rather then doing this in the {lang}.json files we should provide fallback functionality either in languages.js or as part of compile in index.js.