I was testing Ubuntu 16.04 and trying out Libre Office 5 that comes bundled with it, when I noticed that there were no dictionaries - this is presumably an Ubuntu packaging problem as I cannot believe that anyone would normally expect a word processor to have no functioning spell checker. To try to fix the problem I went to extensions manager, downloaded a dictionary and added it. I also followed the link about how to install extensions but the document didn’t seem to correspond to the reality; the illustrations were all for version 3 and clearly something has changed. It suggested that I would be prompted if I wanted the extension to be public rather than private but I got no such choice. The extension did install locally and as far as that one user was concerned did fix the problem once I restarted Libre Office.
I should add that, since asking this question, I have worked out what the actual Ubuntu problem was: it turned out that for some reason (undetermined) the installation had not completed the system wide language support (System Settings->Language Settings) completing this solved my problem. However I still think there is a problem with the Libre Office documentation.
