[6.x] Add missing English translation JSON file

[lazerg]

Fixes #14096

Add the missing lang/en.json file with English translations. For the English locale, the translation keys and values are identical.

Includes a test to verify the file is valid JSON and properly symmetric.

[jasonvarga]

Thanks for the PR! The actual bug is in JavascriptComposer::translations(), which overrides the frontend's English key-fallback with the fallback locale's translations. Fixing that at the root in a separate PR instead of maintaining lang/en.json by hand - the whole point of json based translations is that you don't need a json file for English. Closing this in favor of that — appreciate the repro, it helped find the cause.