Well with your work experience I think you understand the whole issue better than me.
THe English is the 'official' version. For translation, for the LAs, we see the English, and we have an editable field for each localization. You are correct they could make (may have) a seperate localization for American English which would be seperate from the base verbage...but I think this would make things more complicated. As you state, there are keys. I think the American English is then the 'default' or 'master' or something along those lines...the key point being that only BBs (and maybe some special GMs for all I know) can ever edit it. It is the official and in case of rule confusions etc., the translations/localization wording does not count, everything goes back to the core US English, hence why all GMs and LAs must speak English fluently.
I guess its kinda like when the French ran England and all official laws were French/Latin.
Maybe they should hire you to do a viewer update. Sounds like you got the experience and so forth. Maybe they could offer you a few seasons/years of supporter in lew of your work if they are too broke to give you cash for it. I guess it depends on how developer friendly the code for the viewer is. To me adding a few extra keys, a few lines of text, and some kind of functionality to randomly interpret a given event (which would be what the viewer reads from the GE) in multiple different ways, doesn'T sound too complicated...but then again maybe its a lot more complicated and time consuming than I imagine.
I'd be godawful at anything like the viewer to be honest - I could probably help revise the localization if needed but I'd have to think they've probably got little problem with that part. Haven't they said in the past that the viewer was sort of designed by someone external and they can't really update it? That's probably more of a problem - if that wasn't designed in a way that allows for that sort of extensibility then it really would require a total rewrite. Faking a new language to allow re-localizing the master would be a workaround, but if the viewer has the keys hardcoded in it, for example, it wouldn't be possible to add new keys easily.
It's all moot anyhow - I can't imagine this would be high on the priority list. But it's actually fun for me in that way where talking about something you know and almost nobody else cares about is. ;)