The typical scenario is this—you've got some new software and, at some stage, either early or later, some essential thing doesn't work. Either it never worked in the first place or it suddenly stops working. The only thing to help you is an obscure message, such as the one I got about 'libdvdcss'. Then you start solving the problem, by using Google. In the end, you do solve it, even though there are many moments when you think you never will, even though it sometimes takes days of frustration. However, in each case, I've found that the process goes more or less as follows, here I'll use this specific problem as an example and hope it will help others (when they're Googling too):
Pity you can't watch movies though.
And, at the climax (or one of the climaxes since this movie has so many), I figured it out (or I believe I did and by now I'm sure of it): for some reason (my fault? Ubuntu's? a random something else?) when I installed the movie players, I didn't have permission to use them, because I installed as root. OK, that doesn't make sense. But, if I don't set 'sudo chmod 777' on each of the movie player's executables, I get an error message, sometimes about region settings and sometimes about 'libdvdcss'. I'm sure that several of the steps I took above were necessary, but like many others out there in Googleland, they just weren't enough. You need to set permissions on the installed movie players, otherwise you will not be able to watch DVDs in them. That's just the way it is, unfortunately.
And, well, anyway, the above 5 stages are just typical of this kind of process. It's always a small fluke somewhere that makes it all work. (There's also a 6th stage, by the way, one which I'm not looking forward to, called "The Regression".) And I'm not blaming anyone (not Ubuntu, not movie players, not regional settings, and not Anthony Hopkins, who is as likely to be at fault as anyone else). I guess that's just the way it is in today's world. Travel with Google and you'll always arrive where you wanted to go, but the route there is always a scenic one.