The settings and switcher show en_US, and it would be nice to change them to showing en_GB only if you are in the UK.
Download the current GB English version from http://en-gb.wordpress.org/ and untar.
Copy the following files from wordpress/wp-content/languages/ to <your_server>/wp-content/languages/:
In wp-content/plugins/wp-native-dashboard, replace all instances of en_US with en_GB - there should be 7 of them.
Flag and CSS edits are not needed because the entries are already there.
In wp-config.php, edit:
or the language you wish the user-facing pages to be in, eg for Welsh:
Refresh the Native Dashboard settings - you should now see GB English listed as an installed language, and it should be appearing in the language switcher in the top left-hand corner, but US English should have disappeared.