1) You should fully Americanize all the spellings. It is American English people want.
2) This would be far too boring for me to stick with if it is things like "I went to the cinema yesterday". A coherent story, or even better, a domain-specific lesson, would be a much more engaging way to teach a language. I was able to learn when it was "talk to my driver about the security situation and drive plan", but never cared enough for casual conversation. I am usually happy to talk to people who speak horrible English about things I care about, which presumably for the hn audience is tech, startups, etc., but not about sports (cricket!?). If you could do a vertical-specific sayafter.me it would be awesome.
I agree with te_chris, it is not only arrogant but incorrect to make that assumption. I don't actually mind using 'color', but you live in a small world if you think that only a certain accent is relevant.
(I prefer South African English, personally, but I think the commercial market is strongly in favor of American English.)
Same thing in China and Thailand; I don't know about other countries.
More anecdata for you.
"No, your anecdotal evidence is wrong."
"My anecdotal evidence is right."
(A totally fluent UK/IE/etc English speaker probably is fine in any context, but I don't think someone learning from a webapp is starting from that point. If you are going to be a moderate speaker, it is better to be moderate in the target accent.)