I think i must be missing something.
When I go to Option Groups /civicrm/admin/options?reset=1
and choose Language
i see no value listed as Default, and no option when I click 'edit' on an option to set it as Default, yet if i look in the DB for values where option_group_id = 75 it shows en_US as default.
I set this to the desired value but where I am being blind in terms of doing this via the UI?