I dunno, i wouldn't trust too much RM and JHope "fitting in" attitude(Jhope is a good actor, he always tries to "fit in" no matter where), if you look at their facial expressions(i do because i'm biased to them they are my fav) they looks more annoyed than the rest, also JHope is perfectly capable of understanding and talking english (he did so in some fancam), same for Jin and probably V and Suga too, but they avoid it on purpose, pretending to know less than they do.
I'm not saying they don't want success in america, of course they want, but they won't chasing it, they won't change their concept for something they don't consider something long-lasting.
The american fanbase is loyal to americans, not on foreign groups, unless the group is from England/english-speaker-countries(and even like this it's rare stuff), BTS will remain Kpop, even if they will talk in english they can't afford to become something they are not and alienate their strongest and more loyal korean/asian fanbase.
The reason why they avoided the after party in my opinion was not because they don't know english( half of them can totally understand and comunicate) but because they fear americans "celebrities", they are aware they can't trust it, if someone tried to aproach them and took fotos, gossip would start and we know how gossip are serius business in Korea( just look at the totally innocent Suga recent gossip..).
RM and really everyone in BTS are smart, i'm sure RM knows very well that at the moment they are just "easy retweet" matherial in america, most of the fanbase are anyway teenage girls that will move on to the next hot thing soon, if they aimed so much for america they should have done it now or never, but they haven't, they are not risking their loyal fanbase for casual fans.
All their video and Vlive are always in koreans, and JHope didn't even know "hixtape" meaning (doubt he was lying, no reason to do so) showing he never even watch english tweets/tags about him ( and he only thanked the korean tag for his birthday), only RM randomly tweets in english, and not even that often.
If RM is really interested in America(i doubt it ) he'll aim at it after the BTS experiences, not now.
Personally i think like this, we will see with their next comeback if they will have hit songs in america.