OpenStreetMap en-US translation

We cannot translate here to either varietey of en. I think, it is up to you which dialect you choose to write the original messages. Does that answer the question?

Probably not quite. Can or should you be using the language codes en-GB, or en-US, or plain en in your Message file(s)? Frankly, I do not know. Maybe, you can leave en empty and make it use a fallback of en-US, but that should of course be tested.