There’s no way to do this - in general, if you look throughout the interface of OS X, you’ll find that American spellings are used: “Color”, “Center”, and so on. I can’t think of a single program UI that has been localised into British English, and certainly the main OS X UI hasn’t been. A British English version of Scrivener would therefore be incongruous with the rest of the system. In fact, an early version of Scrivener did use British English throughout the interface as standard (with no American English version available), but I ended up moving to American English for consistency. (I’m the developer, and I’m English, by the way.)
Besides, shouldn’t you be asking for a Welsh interface?
Thank you for the rapid reply—flattered to get something from ‘the man himself.’
Thanks for the clarification and providing the reasoning, though a “British English version that is incongruous with the rest of the system,” does sound delicious!
I did have a Welsh version of another word processor. Problem was, it only ever wanted to write about rugby, or how it wanted to hook up with a Kernow version, and separate from the rest of the Operating System.