The Culture Map by Erin Meyer provides good examples of British understatements and how they would be misunderstood by a Dutch person. There are two things that an alien is not allowed to do: a) use an understatement like a native with the same elocutionary force and effectiveness (would "That's an original point of view" ever be understood as "I don't like it, change it" when uttered by a non-native speaker of RP?) and b) mishear or misunderstand like a native speaker does (witness radio presenters), which would fail a candidate in an IELTS oral. Lack of directness has led to plane crashes. "Not a bad thesis topic" (ie very good!).