As for the decimal mark (comma), it's most of the world really. This has nothing to do with grouping in numbers. I find both types of thousands separator (, or .) misleading, pointless and not nice at all. But maybe that's because I grew up in a country that used neither of those.
Then prepare to have your mind blown (if you haven't heard about this before): the correct culture-agnostic term is "grouping separator" because not everyone uses 3 digit groups - in some East Asian languages (specifically Japan and China, not sure who else) you have separate words for "ten thousand" and use 4 digit groups.
I think I have genuinely never seen scientific notation used in an accouting context. If it can be rounded, then we will use 1MM or 1B.
I see it as follows. Unlike science, where there are a few numbers related in complex ways, accounting has very many numbers, related in very simple ways.
wait wait wait, a billion means two different things in Europe and America? I'm from Europe and I always used 1 billion to say 10^9. I hardly see how I would not have noticed figures being so different in say, news articles from America and Britain.
When I was growing up in Ireland, a billion was a million million in Britain and Ireland. Now, we have adopted the American thousand million. In Germany and Italy, a thousand million is a Milliard, and a billion is a trillion.
