translation to UK English?

Probably an easy one to fix…my WordPress site is in English (UK) rather than English (US), and all the time I’m getting a notice (see image) suggesting I might want to translate it into my own language. I do not want to change my site to US English to get rid of it, and wondered if you could either

1) make the minor corrections needed to the converting of the language (aetiology, organise, colour, etc.) with a UK dictionary file;
2) treat the two languages as the same, as in, just as if all English is already translated, or;
3) tell me how to get rid of that warning thing, to stop it annoying me.

Thanks so much, really appreciated.

The page I need help with: [log in to see the link]