Stupidly I created an Odoo v10 community edition database with the language set to "English" and not "English (UK)".  I had got around it by loading a translation and making that the preference for all users and partners, but for some reason I was still unable to deactivate the "English" language via the menu options.

After some digging, I ran the following SQL against the database using psql on the command line:

update res_partner set lang = 'en_GB';

I was then able to deactivate the "English" language, which means that "English (UK)" is now the only active one.  I noticed that there were two hidden partners: Template User and Public User, which were obviously the ones preventing the change earlier.

I took this approach because I did not want to start afresh with a new database, for pretty obvious reasons.  Have I covered this off now, or is there anything else that I should look to change?