British English or American English spelling?
Duolingo will need to decide whether it goes with one or the other. I often get my quite correct British English spelling changed by Duolingo participants as Americans regard certain spellings as 'typos'. This includes for example, 'practising, licensing' with an 's' yet the noun is 'practice, licence' with a c - both are quite the opposite to American English. Other words are 'colour' or 'favour' and so on which in British English take the 'u'. The internet is dominated by American English because of the US's role in developing the technology. If I see American spelling I don't change it to British, but if I translate, I use British spelling. Any comments?
You can add "metre" and "theatre" to the list, and there are a few other words that pop up ("whilst" comes to mind; it's rarely if ever heard in the US, but I see it here quite often). When I first started Immersion, I wrongly assumed that US English was the standard here, including the format for dates, and I edited accordingly. I don't do that anymore. Most Americans do not encounter British spelling on an everyday basis, so it really does look wrong to them. Now that I'm spending a great deal of time here, I often see British English and it looks just as correct to me as US English. I never edit it when I see it, just as I hope that my US English won't be edited simply for being US English.
Duolingo could allow people who upload articles to specify which variety of English they would like others to use when translating the article. This could be clearly posted at the top of the article, which might help to achieve consistency within an article. Meanwhile, I tend to do what you do, which is use US spellings for my own translations. If I edit someone else's translation, and the translator used British spellings and/or date formats, I do not change that part of their translation.
Specifying the variety of English is a good idea. However, those who use British English seem more aware of American spelling than the other way around, particularly as computer spell checkers have been developed in the US. I've just explained it each time someone changes my spelling to US English and one person was really pleased I'd pointed it out as he had no idea there was a difference. That fits in with what you said. C'est a vie :-).
I haven't gone on the translations here, since I literally just started and I can barely speak German, but I would just leave the American English alone, and I would translate in British English, since that's what we use in Canada. American English doesn't look wrong, since I encounter it all the time online, but it looks less correct than British English. Duolingo teaches American English, so technically American English is the 'correct' English in those forums, although it shouldn't matter too much.