Ok so I'm going to criticize my own PR here because I just noticed that this particular string is not taking from the translation file .. *facepalm*

While the apply_coupon_code API call should always return an error message, this message is just a fallback in case there's some sort of fatal error that causes the api call to blow up or if an error is omitted.

What's the standard for managing translations in javascript files? I did a quick runthrough many JS files and couldn't find anything that would give me a clue.