FWIW I’m not quite convinced there’s that much of a dialectical divide: “Not bad,” “he’s not wrong,” etc. sound entirely natural to me in American English.
"American English" has so many dialects and regional variations that aren't even mutually intelligible that making statements about it is pointless anyway.
I'd argue there's few Americans I flat out couldn't understand, even if it sounds like they're putting their words through a blender. And I say that having lived all over the country, Northeast, Midwest, West, and deep South. Accents can be thick but they're largely intelligible. Unlike, say, the Scots.
Especially compared to a language like German. I took 5 years of German and still didn't have a damn clue what anyone was saying if they were talking in dialect.
In a scale of 0 to 10 where "bad" is 0, one side will take "not bad" as a 4~5 while the other side meant it as 7~8.