Wikimedia Bugzilla is closed!

The page content block, and the textarea for editing, should support having a language variant set for the page.

e.g
a tag in the page, like {{LANGUAGE:en-GB}}
should set lang="en-GB" on the content div, and on the textarea wpTextbox1.

Browser spellcheckers will then use the correct spelling definitions for that page.

On En.Wp there is already templates which could implement this, and this would support the many policies of retaining existing spelling.

https://en.wikipedia.org/wiki/Template:Use_British_English