v1.3: Don't lookup non-existent locale's in the i18n module

Status: **Opened for visibility**

Reviewers: ** @jansepar @rrjamie @noahadams **

## Changes
- Guard against looking up non-existent locales in the i18n function

## How to test-drive this PR
- Build yourself a mobify-client using this commit for mobify 1.3
- Create a project where you initialize an i18n module
- Add a locale to the i18n module which doesn't exist in the json source file
- Try looking up a key, it should return the value of that key under "DEFAULT" instead of erroring