When British people say something [idiomatically], do other nationalities, whether English-speaking or non English-speaking, always understand accurately what is implied by the words being used? Communications between people using supposedly the same language, English, as their native tongue can certainly be fraught with misunderstanding when British, Americans or Australians are involved, although I'd say this tends to be marginally less so when I've dealt with Canadians. My own experience over the years has led me to understand that very often misunderstandings arise because people (in this case we British) do not necessarily say directly what they mean, out of 'politeness' or an unwillingness to upset the other person in a social or business context. Mind you, it is not only we British who do this - my dealings with non-European nationalities such as Chinese or Japanese have often thrown up similar mutual misunderstandings and for much the same reasons, not to mention the perils of taking at 'face value' what a French person really means when [s]he says something.
Anyway here is what I came across in the Twitter feed of someone I follow:
I asked him where he got it from and he mentioned the website of a UK charity, but it seems they got it from someplace else and so on - this article in the University of Pennsylvania's "Language Log" gives more background although how accurate or exhaustive it is I cannot say. It adds some more detail to the table above and gives a few phrases in French which can mask what is really being said. In my experience all these are scarily accurate.